Abstract
Introduction
Method
Theory
Application
Limitations
Results
Schiller-Schickard Area
Area West of Mare Humorum
Area West of Oceanus Procellarum
Conclusions
References
Tables
Figures

de ~ 0.1 Dtc

(1)

This equation is generally considered to be valid for both simple and complex craters [e.g., Melosh, 1989, p.78]. In order to relate this equation to measurable crater parameters, a relationship is needed to convert the transient crater diameter (D_tc) to rim-crest diameter (D_r), which is measured for the craters of this study from Lunar Orbiter images. From Melosh [1989, p.129], we note that for simple craters

Dtc = 0.84 Dr

(2)

Substituting into equation (1) gives

de ~ 0.084 Dr

(3)

For complex craters, Croft [1985] shows that

Dtc ~ DQ0.15 Dr0.85

(4)

where D_Q is the transition diameter between simple and complex craters. For the Moon, D_Q is approximately 19 km [Pike, 1980]. Thus,

Dtc ~ 1.5 Dr0.85

(5)

Substituting into equation (1) gives

de ~ 0.15 Dr0.85

(6)

The error associated with these equations is approximately 15%.
       These equations were applied to the dark-haloed crater data set, in order to determine the excavation depths of these craters (Tables 1, 2, 3). The excavation depths were used in thickness calculations and average thicknesses were determined for each cryptomafic unit. The volume of cryptomafic material was determined by multiplying this average thickness by the total area of the cryptomafic deposit.


        Limitations
       Several uncertainties exist in the thickness and volume estimates that are calculated with this approach. These uncertainties are generally related to the relatively small population of fresh craters that are superposed on the areas of interest, and the nature of crater and basin ejecta deposition processes.
       First, the largest dark-haloed craters that are observed in any given area might not necessarily be reaching the bottom of the cryptomafic unit. Penetration to the bottom of a cryptomafic deposit can be confirmed by the identification of large, relatively fresh craters in the cryptomafic region that do not exhibit dark halos. Such craters excavate highland material from below the cryptomafic layer, obscuring the dark halo signature (Figure 3). We found no evidence for such large, fresh craters in any of the three study areas, therefore, the possibility that the bottom of the cryptomafic deposit is not being reached must be considered. An examination of our equations above shows that dark-haloed craters on the order of at least 10 km in diameter would be required to sample to a depth of 1 km, and craters 30 km in diameter would be required to excavate to a depth of 2 km. None of the dark-haloed craters noted in Tables 1, 2, and 3 are larger than 22 km in diameter; thus these dark-haloed craters would not be reaching to the bottom of a cryptomafic deposit whose base is 2 km below the surface. Conversely, several of the dark-haloed craters are larger than 10 km in diameter, and if the bottom of the cryptomafic deposit is positioned only 1 km below the surface, these dark-haloed craters should be tapping this base. In such a case, our inability to find fresh craters that excavate material from the sub-mafic layer could simply reflect a paucity of fresh craters in this size range. Clearly, the question of whether or not dark-haloed craters are reaching the bottom of a cryptomafic deposit must be addressed for each study area.
       Secondly, the cryptomafic deposit, as defined by dark-haloed crater sizes, theoretically includes only that material which remains a part of the distinct volcanic unit. Some mafic material can also be found in the overlying regional ejecta, incorporated during the ballistic sedimentation process which emplaced this ejecta deposit [Oberbeck, 1975]. Our estimates of ancient volcanic volumes will lack the input of this component, since it has been removed from the upper portions of the cryptomafic unit and incorporated into the overlying regional ejecta deposit, making it undetectable by this technique.
       Finally, the depth of excavation (d_e) from dark-haloed craters may not be an exact measure of the stratigraphic layer thicknesses. Consider Figure 6, where a minimum dark-haloed crater and a maximum dark-haloed crater are schematically illustrated. The minimum dark-haloed crater represents the smallest possible dark-haloed crater that can form when enough mafic material has been excavated to make the dark halo just barely visible. Clearly, the depth of excavation must penetrate into the mafic substrate by some depth (d_m) before sufficient volcanic material is excavated to produce this barely visible dark halo. Thus, the depth of excavation does not measure the exact thickness of the overlying ejecta (t_m), but rather the ejecta thickness plus the value d_m. The maximum dark-haloed crater in Figure 6 represents the largest possible dark-haloed crater that can form, where the dark halo is still barely visible despite the fact that it is partially obscured by highland material. Here, the depth of excavation can penetrate into the underlying highland crust by some maximum depth (d_h) before sufficient highland material is excavated in order to completely obscure the dark halo. Thus, the depth of excavation measures not only the thickness of the overlying layers (t_h), but also includes the value of this penetration depth (d_h). Photometric theory predicts that a small quantity of dark material in a bright medium has a greater impact on the albedo of the mixture than a similar amount of light material in a dark medium [Hapke, 1993]. Thus, more light material would be required to obscure a dark halo than dark material would be required to form one. The value of d_h is also somewhat dependent on the thickness of the mafic deposit, which affects the excavated volume of dark material that will need to be obscured. Estimates for the magnitudes of d_m and d_h will be discussed in Chapter 2.
       In summary, several factors suggest that our thickness, and thus volume, calculations will produce underestimated values: the mafic component incorporated in the regional ejecta deposit, the depth of penetration into the volcanic layer (d_m) before an observable dark halo is formed, and the possibility that the bottom of the cryptomafic deposit is not being reached in some cases. On the other hand, if the bottom is being reached, the depth of penetration into highland substrate (d_h) required to obscure a dark halo, will act in the opposite direction, causing the volumes of cryptomafic material to be overestimated. Clearly, the limitations of this technique must be considered for each region where it is applied.



Results

        Schiller-Schickard Area
        The Schiller-Schickard area is located approximately 1400 km southeast from the center of the Orientale basin, between the craters Schiller and Schickard (Figures 1, 4, 7). It is believed that Orientale ejecta (Hevelius Formation) obscures a cryptomafic deposit in this area [Mustard et al., 1992; Antonenko et al., 1995]. However, some sections of the Schiller-Schickard cryptomafic region may also include subsequent mafic patches that were emplaced on top of the Orientale ejecta deposit and then obscured at some later time by "dusting" from crater ejecta. The existence of cryptomafic deposits that have formed by obscuration from crater ejecta has been previously proposed in the Balmer basin region on the eastern limb of the Moon [Hawke and Spudis, 1980]. It has been suggested, on the basis of crater dating [Greeley et al., 1993], that the region due south of crater Schickard, which is known as the Schiller-Zucchius plains and is particularly smooth and dark, may represent this kind of cryptomafic deposit [Head et al., 1993]. Such a unit, since it post-dates the Orientale event [Greeley et al., 1993], is expected to perch on top of the Orientale ejecta deposit, allowing a potentially larger cryptomafic unit to be contiguously connected beneath (Figure 2). The Schiller-Zucchius plains, however, comprise no more than 6% of the area of the Schiller-Schickard cryptomafic deposit, as estimated by Mustard et al. [1992], thus, the presence of a distinct cryptomafic unit in the Schiller-Zucchius plains is expected to have a minor effect on the significance of our volume estimates.
       Forty-two (42) dark-haloed craters (Table 1) and fifty-six (56) non-dark-haloed craters were identified in the Schiller-Schickard area (Figure 7). The dark-haloed craters are mainly concentrated within an area around the large craters Schiller and Schickard, with non-dark-haloed craters generally occurring outside of this region. Several non-dark-haloed craters occur within the dark-haloed crater area, but these are concentrated in areas that appear to be isolated topographic highs (for example, crater rims and rim remnants) around which the cryptomafic volcanics flowed, forming kipukas. For example, two non-dark-haloed craters are seen on the rim of Schiller crater (in Figure 5b). These appear to have excavated the highland rim and ejecta deposits of Schiller crater, which predates the cryptomafic emplacement event. Non-dark-haloed craters, therefore, not only help in defining the margins of a cryptomafic deposit, but can also aid in determining the geometry of the deposit interior.
       The distribution of the dark-haloed craters and non-dark-haloed craters, along with the shape of the cryptomafic unit they define (Figure 7), suggest that the Schiller-Schickard cryptomafic deposit represents the filling of an old, degraded basin. This interpretation is supported by Clementine topography data [Zuber et al., 1994], which shows the presence of a shallow circular depression between Schiller and Schickard craters. Furthermore, Clementine gravity data [Zuber et al., 1994] suggests the presence of a possible weak mascon here. However, there is no documentation for the presence of a basin at this location. The Schiller-Zucchius basin [Wilhelms, 1987] represents only a minor portion in the southwest region of the area we suggest, and the recently proposed Schickard-Mee basin [Blewett et al., 1995] overlaps with only a small fragment in the northern part of our cryptomafic area. It would, therefore, appear that the basin suggested by this cryptomafic deposit has not been previously identified.
       Depths of excavation (d_e) were calculated for each dark-haloed crater (Table 1) in order to determine the thickness of the cryptomafic deposit. The first step was to estimate the thickness of the bright layer that overlies and obscures the cryptomafic unit in this region. The Orientale impact basin is the prime candidate for the source of this obscuring layer. Therefore, the radial distance between each dark-haloed crater and the center of Orientale was calculated (Table 1). The dark-haloed craters were sorted in order of increasing distance from Orientale and grouped into bins representing 100-km distance intervals. This allowed the smallest and largest craters in each 100-km bin to be identified.
       The smallest dark-haloed craters in each 100-km distance interval are plotted in Figure 8 as a function of the distance from Orientale. Column heights represent the estimated thickness of the overlying layer, as determined from the depth of excavation of the smallest dark-haloed craters. These data suggest that, in the ~900 km-wide area that was analyzed, the overlying layer ranges from <200 meters to 600 meters thick, with an average thickness of approximately 260 meters. If the overlying layer consists primarily of Orientale ejecta, then the layer thickness is expected to decrease as a function of distance from Orientale (Figure 3). Theoretical decay curves for the thickness of Orientale ejecta, calculated from the equations of McGetchin et al. [1973] and Schultz et al. [1981] for a 900 km and a 620 km Orientale diameter [Wilhelms, 1987], are shown in Figure 8. Estimates of the overlying layer thickness appear to be significantly greater than theoretical predictions for ejecta from Orientale and the data do not appear to be consistent with an exponentially decaying ejecta curve. Variance of the data with respect to the mean is significantly less than the variance with respect to either of the theoretical ejecta curves, suggesting that the data is better fit by a straight horizontal line. This fact may be related to the limited resolution of the data set, which could result in the smallest observed dark-haloed craters not necessarily being the smallest possible. In this case, the thickness estimates of the overlying deposit will yield maximum values, and thus give minimum estimates for the thickness of the cryptomafic unit. Conversely, possible explanations for a consistently thick overlying layer include an error in the estimate due to the limitations of this technique, the presence of an anomalously thick deposit of Orientale ejecta in this area [Wilhelms, 1987, Plate 8A], contributions from other ejecta deposits, or contributions from the pre-impact substrate through ballistic sedimentation processes [Oberbeck, 1975]. Even if overestimated, the thickness estimates determined here appear to be consistent with the hypothesis that Orientale basin ejecta is a significant component of the obscuring layer deposit. We propose, therefore, that the cryptomafic deposit here may predate the Orientale basin-forming event.
       Thickness estimates for the cryptomafic deposit were obtained by calculating the difference between excavation depths from the largest and smallest dark-haloed craters in each 100-km distance interval. Because the smallest dark-haloed craters may be overestimating the overlying deposit, these calculations should give minimum estimates. The minimum thickness of the cryptomafic layer, estimated this way, is plotted in Figure 9 as a function of distance from Orientale. The minimum thickness of the cryptomafic layer in this region can be seen to vary from <400 - 1800 meters, with the average being approximately 900 meters. This range and average value are both within the limits of what is typical for mare thicknesses on the western limb [De Hon, 1979; Head, 1982].
       In our analysis of this region, we did not find any craters that could definitely be identified as tapping into highland substrate from beneath the cryptomafic layer. There is no positive evidence to indicate that the largest dark-haloed craters in this region are penetrating to the bottom of the cryptomafic deposit. Furthermore, the thickness of the smallest dark-haloed craters may be overestimating the thickness of the overlying highland deposit. These thickness estimates should, therefore, be considered as minimum values. It should be noted that the choice of bin size affects our thickness estimates. For example, larger bin sizes would include more craters and so statistically should give higher thickness values. However, it is possible that some of our large craters may be penetrating to the bottom of the cryptomafic deposit. To reduce the statistical significance of such craters, smaller bin sizes are used. This insures that our thickness estimates are conservative. The agreement between these thicknesses and those of typical mare on the western limb suggests that our estimates are relatively reasonable.
       Based on the boundary delineations shown in Figure 7, the area of the Schiller-Schickard cryptomafic deposit was estimated to be 3.8 x 10⁵ km², agreeing well with the estimate of Mustard et al. [1992] which was based on spectral mixing analysis results. This area is relatively large, corresponding to approximately 2.5 times the area of the nearby Mare Humorum [BVSP, 1981]. We multiplied this areal estimate by the average minimum thickness estimate from above to obtain a conservative volume estimate of ~3.4 x 10⁵ km³ for the Schiller-Schickard cryptomafic deposit. In using this average volume estimate, one must always be aware of the factors that affect its value: possible variations in the estimated thickness of the cryptomafic deposit (Figure 9); variations within the deposit interior, suggested by non-dark-haloed craters that most likely represent highland kipukas (Figures 5b and 7); and the lack of craters that penetrate through the cryptomafic deposit to the underlying highland substrate, which we interpret as an indication that our thickness is a minimum estimate. This volume should, therefore, be considered a conservative estimate. Like the areal estimate, the volume estimate of ~3.4 x 10⁵ km³ for the Schiller-Schickard cryptomafic deposit is also relatively large, corresponding to approximately 1.25 times the volume of Mare Humorum [BVSP, 1981]. Comparing these area and volume estimates to those of the known lunar maria [Head, 1975a], the Schiller-Schickard cryptomafic deposit is found to conservatively represent 6% of the total lunar mare area and 3.5% of the total lunar mare volume.
       Several mare patches can be found both within the Schiller-Schickard cryptomafic area and in the surrounding regions (Figure 7). Estimates of the areas and volumes for a majority of these mare deposits have been made by Yingst and Head, [1997], showing that these mare patches are small relative to the cryptomafic deposit. On the basis of their study, we estimated that, together, these mare patches represent an area of approximately 2.1 x 10⁴ km², or less than 6% of the total area of the cryptomafic deposit, and a volume of 6.3 x 10³ km³, or less than 2% of the volume of the cryptomafic deposit. We can state that volcanism in this area was significantly more voluminous prior to the emplacement of the cryptomafic-obscuring layer than following it. Since the Schiller-Schickard cryptomafic deposit is suspected to predate Orientale, which was emplaced 3.84 b.y. ago in the Early Imbrian Period, the stratigraphic marker provided by the Orientale ejecta deposit can be used to indicate that Early Imbrian volcanism may have been volumetrically very significant in the Schiller-Schickard area.


        Area West of Mare Humorum
       The area west of Mare Humorum is located approximately 1100 km east of the center of Orientale, and includes the regions surrounding Mare Humorum, as well as areas slightly further west and north (Figures 1, 4, 10). Spectral mixing analysis [Mustard et al., 1994] has identified the presence of significant mare components in this area, and studies using Earth-based telescopic spectra [Lucey et al., 1991; Hawke et al., 1993] have identified several dark-haloed craters here, suggesting that cryptomare deposits may be found in this area. Furthermore, Orientale ejecta (Hevelius Formation) is prominently mapped in this region [Marshall, 1963; Titley, 1967; McCauley, 1973; Wilshire, 1973; Karlstrom, 1974; Saunders and Wilhelms, 1974; Scott et al., 1977], providing a possible way for ancient mafic deposits to be covered and obscured.
       Fifty (50) dark-haloed craters (Table 2) and twenty-four (24) non-dark-haloed craters were identified in this area (Figure 10), including those dark-haloed craters identified by Lucey et al. [1991] and Hawke et al. [1993]. Unlike the Schiller-Schickard cryptomafic area, the dark-haloed craters here are not distributed in a basin-like arrangement, but rather appear to be more randomly scattered, often occurring within large, old craters. The number of non-dark-haloed craters is rather low, making boundary delineations difficult. However several pieces of evidence suggest that more than one cryptomafic deposit exists in this region. For instance, we find many mare patches, which are often associated with nearby non-dark-haloed craters. These suggest that volcanism in this area was patchy and that cryptomafic volcanism, as indicated by the dark-haloed craters, may have also been patchy. Also, several mare-filled craters are found in the region and some outlying dark-haloed craters occur within old subdued craters. These suggests that volcanism in the region was often confined to a single crater and that some cryptomafic deposits may also be confined by older craters. All of these findings suggest that those dark-haloed craters furthest from Mare Humorum represent small isolated cryptomafic patches instead of one large, contiguous cryptomafic body. Conversely, the spectral mixing analysis of Mustard et al. [1994] and Mustard and Head [1996] suggests that the dark-haloed craters closest to Mare Humorum represent a contiguous cryptomafic unit. This large cryptomafic unit may be an extension of early volcanics that were deposited in the Mare Humorum and southern Oceanus Procellarum areas prior to the Orientale basin event.
       With these observations in mind, the dark-haloed crater and non-dark-haloed crater data, along with spectral mixing results [Mustard et al., 1994] and various topographic cues (such as light plains and elevated topography interfaces), were used to map the boundaries of the cryptomafic deposits in this region (Figure 10). Lone dark-haloed craters that are not located within a larger crater (such as those found south and west of Byrgius crater) are interpreted to represent only very small cryptomafic patches. Those dark-haloed craters that are located very close to mare patches (such as those to the southwest of Mare Humorum) are interpreted to represent very minor cryptomafic extensions to their proximal mare patches. Several lines of evidence, such as the exposure of a large highland area around the crater Mersenius, the presence of a non-dark-haloed crater within the primary cryptomafic area, and the shape of the main, western boundary of the cryptomafic deposit, suggest that these early volcanics were emplaced in an area of rough, heavily cratered, topography. The main cryptomafic body may have resulted from the flooding of a degraded Humorum basin, where the early flows spread out well beyond the extent of the current Mare Humorum area. Further evidence for this interpretation is seen in Clementine topography data [Zuber et al., 1994], which shows the presence of an irregular and shallow topographic low around the Mare Humorum area. Also, part of the cryptomafic deposit appears to be aligned concentrically around Mare Humorum, within the Humorum basin rim [Wilhelms, 1987], again supporting this interpretation.
       To determine the thickness of the cryptomafic deposit, a similar approach to that used for the Schiller-Schickard region was applied here (Table 2). In the area west of Mare Humorum, Orientale ejecta is suspected to be the prime obscuring material for the cryptomafic deposit, thus dark-haloed craters were again assessed as a function of distance from Orientale. The smallest dark-haloed craters in each 100-km distance interval are shown in Figure 11, representing the estimated thickness of the overlying layer. The exponential lines represent theoretical decay curves for Orientale [McGetchin et al., 1973; Schultz et al., 1981], for a 900 km and a 620 km Orientale diameter [Wilhelms, 1987]. Estimated thickness values range roughly from 150 meters to 800 meters, with the average being ~300 meters. Comparing this data to the theoretical decay curves shows that the obscuring layer falls within, or slightly above, the range predicted for Orientale ejecta, but again the data is not statistically consistent with an exponentially decaying ejecta curve. If Orientale basin ejecta is a major component of the obscuring layer, then contributions from other ejecta deposits, such as Letronne, the Fra Mauro (Imbrium) and Vitello (Humorum ) Formations [Mustard and Head, 1996], may be required to explain the lack of an exponential decay. The inclusion of such ejecta deposits in the obscuring layer would have profound implications for the age of these cryptomafic units, since the cryptomafic deposits must predate any of the impact events represented in the obscuring layer (see Figure 13). Conversely, it is possible that the limited resolution of the data set does not allow the smallest dark-haloed craters to be observed, yielding overestimated thicknesses. Even if overestimated, these thicknesses appear to be consistent with the hypothesis that Orientale ejecta may be a significant component of the mafic-obscuring layer in this area. Thus, the cryptomafic deposits west of Mare Humorum should predate the Orientale event and so may potentially be Early Imbrian in age.
        Thickness estimates for the cryptomafic deposits of this region are presented in Figure 12. These estimates range from roughly 60 to 1200 meters, with the average being approximately 600 meters. Both range and average values are within the limits of thicknesses typically observed for mare deposits on the western limb of the Moon [De Hon, 1979; Head, 1982]. Again, because the smallest craters may overestimate the thickness of the overlying deposit, these thicknesses may represent minimum values.
       In our analysis of this region, we did not find any large, young, non-dark-haloed craters that appear to penetrate to the bottom of the cryptomafic deposits. However, the Earth-based spectral studies of Hawke et al. [1993] have identified a highland component in the center of one dark-haloed crater, Gassendi G. They interpret this to indicate that the dark-haloed crater Gassendi G has just barely penetrated to the bottom of the cryptomafic deposit. Gassendi G is a 7.8 km diameter crater, located ~1430 km from the center of Orientale. Our calculations show that the depth of excavation for Gassendi G is ~650 meters, indicating a potential mafic thickness of ~400 meters at this location. This estimate is comparable to the thickness value that was previously estimated at this distance from Orientale (Figure 12), because Gassendi G is almost the same size as the largest dark-haloed crater that is found in this distance interval. Therefore, if the assessment of Hawke et al. (1993) is correct and Gassendi G has penetrated to the bottom of the cryptomafic deposit, this would suggest that the largest dark-haloed craters in this region are reaching the bottom of the mafic units. However, considering the potential overestimation of the overlying highland deposit, these thickness estimates are still expected to represent minimum values.
       Based on the boundaries defined in Figure 10, area estimates for the cryptomafic deposits in the region west of Mare Humorum were calculated. Those cryptomafic deposits that are represented by lone dark-haloed craters or craters near mare patches were considered to be relatively small and thus were not included in the following calculations. For the rest of the cryptomafic deposits, their total area was estimated to be ~2.3 x 10⁵ km², corresponding to 1.5 times the area of Mare Humorum [BVSP, 1981] or ~3.5% of the total exposed lunar mare area [Head, 1975a]. Multiplying this areal estimate by the average minimum thickness of the cryptomafic deposits (600 m) gives a conservative volume estimate of ~1.4 x 10⁵ km³, which equals ~50% of the volume of Mare Humorum [BVSP, 1981] and approximately 1% of the total lunar mare volume [Head, 1975a].
       Many mare patches can be found in the region west of Mare Humorum. Most of these have not been considered by the study of Yingst and Head, [1997]. Therefore, we estimate their areas from lunar maps [Marshall, 1963; Titley, 1967; McCauley, 1973; Wilshire, 1973; Karlstrom, 1974; Saunders and Wilhelms, 1974; Scott et al., 1977] and multiply these by the average pond thickness from Yingst and Head, [1997] to obtain an estimate of their volume. These mare patches are found to represent an area of 1.1 x 10⁴ km² and a volume of 8.4 x 10³ km³, or approximately 25% of the total surface area and ~6% of the volume of all cryptomafic materials located here. Again, the bulk of extrusive volcanism in the area west of Mare Humorum appears to have occurred prior to emplacement of obscuring material. Furthermore, mare volcanism appears to have been more significant west of Mare Humorum than in Schiller-Schickard, as is indicated by the larger surface area of mare patches. The presence of Mare Humorum directly east of this area shows that the bulk of volcanism here actually occurred after the emplacement of the obscuring deposit and took place in the interior of the Humorum basin. Clearly, the stratigraphy of this area is considerably more complex than was first stated. For example, Mustard and Head [1996] have proposed a stratigraphic column for the region southwest of Oceanus Procellarum (Figure 13). Here, they observe that the 100-km diameter crater Letronne has a low-albedo rim deposit, which contains a significant component of mare material in its ejecta. On the basis of this observation, Mustard and Head [1996] propose that this region consists of large quantities of early volcanic deposits, which are characterized by the interleaving of alternating mare and highland ejecta units (Figure 13). If their interpretation is correct, then the dark-haloed craters of this study may only be penetrating through the top-most cryptomafic layer and it is possible that further volcanic deposits may be found at greater depths. In this scenario, the highland component identified in the center of Gassendi G by Hawke et al. [1993] could correspond to the Letronne ejecta deposit. Our data can neither support nor contradict this interpretation, since none of the dark-haloed craters or non-dark-haloed craters near Letronne are significantly larger than Gassendi G and thus provide no information on the stratigraphy below the highland material identified in Gassendi G [Hawke et al., 1993].


        Area West of Oceanus Procellarum
       The area west of Oceanus Procellarum is located approximately 950 km north-east of the center of Orientale basin, occupying the area near the shores of Oceanus Procellarum, between the craters Grimaldi and Einstein, with some extensions further west (Figures 1, 4, 14). Spectral mixing analysis of Galileo SSI data [Mustard et al., 1994; Mustard and Head, 1996] has identified the presence of a moderate component of mare material in a narrow zone along the Procellarum shore. Furthermore, a dark-haloed crater can clearly be seen in this area (the dark-haloed crater indicated in Figure 1a and Orientale ejecta (Hevelius Formation) is prominently mapped here [McCauley, 1967; Scott et al., 1977], indicating that mafic material may have been covered and obscured in this region. These observations all suggest that some cryptomafic deposits may be found here.
       Only nine (9) dark-haloed craters and five (5) non-dark-haloed craters were located in the area west of Oceanus Procellarum. The dark-haloed craters appear to be randomly scattered and often occur in old or degraded craters. On the basis of this evidence, and considering the presence of several small, disconnected mare ponds in this area [Greeley et al., 1993; Yingst and Head, 1994, 1997], we suggest that these dark-haloed craters most likely represent small, isolated cryptomafic patches rather than one large, contiguous cryptomafic deposit. This interpretation is supported by Clementine topography data [Zuber et al., 1994], which shows that the terrain in this area is very rugged, containing many topographic highs and lows, but that there is no overall low feature that might be indicative of a regional cryptomafic deposit. The sparseness of both dark-haloed craters and non-dark-haloed craters presents a problem, making boundary delineations for these small cryptomafic patches very difficult. For this reason, cues from topography and light plains were relied upon. Most of the dark-haloed craters are located within larger craters or within light plains units, allowing these features to help confine the boundaries of the cryptomafic deposits.
       Crater diameters were measured for each of the nine dark-haloed craters in this area, and their depths of excavation calculated (Table 3). The paucity of craters made the approach used for the other regions impractical here. Instead, the smallest dark-haloed crater in Table 3 was adopted for the thickness of the obscuring layer in this region (200 m). This adoption of a single thickness for the overlying layer is feasible for the region west of Oceanus Procellarum, because dark-haloed craters here have a range of distances spanning only 200 km, as opposed to the 900 km range in other regions. An estimate for the thickness of cryptomafic patches in this area can be determined from the largest dark-haloed craters in each patch. Since it is not clear if the bottom of the patches is being reached, these estimates should be considered minimum. The excavation depths of the largest craters in each patch are 2060, 580, 875, 1142, and 1168 meters (Table 3). Using these and the thickness of the overlying deposit, the minimum thicknesses of the cryptomafic deposits is estimated to vary from 380 to 1860 meters, with the average being ~950 meters. This is comparable to the minimum thicknesses of the other cryptomafic regions (average ~900 m in Schiller-Schickard and ~600 m west of Humorum), and is within the limits of typical mare thicknesses for the western limb [De Hon, 1979; Head, 1982].
       Theoretical predictions for the thickness of Orientale ejecta at this location range from 200 - 600 meters (Figure 11, Table 3). The excavation depth of the smallest dark-haloed crater falls just within the lower limit of this range. Therefore, we propose that Orientale ejecta may comprises the bulk of the obscuring layer. This suggests that the cryptomafic patches in the area west of Oceanus Procellarum may be Early Imbrian in age, similar to the proposed ages of cryptomafic deposits in the other areas.
       Area estimates for the cryptomafic patches in this region were calculated based on the boundaries shown in Figure 14. The total area of cryptomafic material was estimated to be ~3.8 x 10⁴ km², corresponding to 0.6% of the total exposed lunar mare area. Multiplying the areal estimate by the average minimum thickness (950 m), gives a conservative volume estimate of 3.6 x 10⁴ km³. The combined volume of cryptomafic patches in this area is, therefore, relatively insignificant, corresponding to 13% of the volume of Mare Humorum and 0.4% of the total volume of lunar maria [Head, 1975a].
       Several mare patches are located in this region. All of these patches have been studied by Yingst and Head, [1997], who estimate that the mare patches in this region have a surface area of 2.1 x10⁴ km² and a volume of ~1.2 x 10⁴ km³. These patches, therefore, represent approximately 56% of the total area of the cryptomafic deposits in this region and 40% of the volume. This observation suggests that extrusive volcanism was equally important both before and after the emplacement of the cryptomafic-obscuring deposit.
       Mustard and Head [1996], however, propose that significant quantities of mafic materials were emplaced prior to the Orientale impact event, in the region that was later to become Oceanus Procellarum (Figure 13). The western edges of these mafic materials would have been reworked and obscured by the Orientale ejecta deposit, which was then embayed by the volcanics that formed Oceanus Procellarum. If this interpretation is correct, cryptomafic deposits may exist beneath the present Oceanus Procellarum as well as along the Procellarum shore, and the volume of early volcanics in this region could be significantly increased. Such deposits are not readily detectable by the technique presented here, since the thicker sections would be covered by large quantities of younger lavas, while the thinner sections, near the shorelines, would have been obliterated by basin impacts or reworked into basin ejecta deposits by ballistic sedimentation processes. Other techniques, as outlined by Antonenko et al. [1995], may be useful in further assessing the significance of such deposits.



Conclusions

       We developed techniques that use dark-haloed craters to study the three-dimensional geometry of cryptomafic deposits, yielding estimates of area, thickness and volume. These techniques were applied to the western limb of the Moon, particularly the Schiller-Schickard region, the area west of Mare Humorum, and the area west of Oceanus Procellarum. Cryptomafic deposits in these regions were found to have average minimum thicknesses of 600 - 950 meters, and a surface area of 6.5 x 10⁵ km², yielding a conservative total volume estimate of 5 x 10⁵ km³. Together these cryptomafic materials correspond to ~10% of the presently exposed lunar mare area and ~5% of the total known mare volume. Even with these conservative estimates, cryptomafic materials on the western limb of the Moon are shown to be significant.
       Our data suggest that the three study areas exhibit different geometries of volcanic emplacement (Figure 15) [Head, 1975b]. The Schiller-Schickard area appears to be an example of a basin-fill cryptomafic deposit, where the infilling of an old, degraded basin, with rough topography, produced an irregularly shaped deposit (Type IA2). The main cryptomafic body in the area west of Mare Humorum may represent a basin-concentric (Type IA3) or irregular (Type IIB) deposit, where the rim of Humorum basin appears to have constrained the shape for part of the deposit, but in other places allowed the flows to spread out into an irregular deposit. The cryptomafic deposits west of Oceanus Procellarum and the remote patches west of Mare Humorum probably represent only isolated crater fill volcanism (Type IIA), where flooding of a relatively small crater produces thin, circular deposits.
       This study may have implications for the sequence and timing of volcanism [Wilhelms, 1987] on the western near side of the Moon. Cryptomafic deposits in the western limb region are areally and volumetrically significant, showing that early volcanism was an important process on the western limb of the Moon. In the Schiller-Schickard area, early volcanism was more important than volcanism that occurred after emplacement of the cryptomafic-obscuring layer. In the area west of Mare Humorum, early volcanism was again more significant than later volcanism, since the volume of mare patches represents only 25% of the cryptomafic deposits. However, large volumes of later volcanics were also extruded in the Mare Humorum basin, just east of this region. The area west of Oceanus Procellarum appears to have experienced generally minor amounts of volcanism both before and after emplacement of the obscuring layer. However, large volumes of later volcanics were also extruded to the east of this region, within Oceanus Procellarum. Generally, extrusive volcanic activity at the time of emplacement of the cryptomafic-obscuring layer was largely waning on the western limb, with the centers of volcanic activity apparently shifting eastward. For all of the cryptomafic deposits in this area, the cryptomafic-obscuring layer is suspected to consist, at least in part, of Orientale ejecta. Therefore, the stratigraphic marker provided by the Orientale ejecta deposit, emplaced 3.84 b.y. ago in the Early Imbrian Period, can be used to suggest that Early Imbrian volcanism may have been volumetrically very significant on the western limb. Support for the existence of Early Imbrian volcanism can be found in the Apollo sample record. Mafic clasts as old as 4.36 b.y. [i.e., Taylor et al., 1983; Nyquist and Shih, 1992] have been identified in returned lunar samples, showing that early volcanism did occur on the Moon.
       On the basis of our analysis of the western limb of the Moon, we propose that, lunar wide, cryptomafic deposits may represent a significant fraction of the total known mare volume. If the total area of known mare deposits represents 17% of the surface area of the Moon [Head, 1975a], and the area of our study region represents ~6.5% of the total highland surface, our findings can be extrapolated to provide a potential estimate of the areal and volumetric significance of cryptomafic units throughout the lunar highlands. In this study region, cryptomafic deposits occupy ~31% of the surface area and represented an addition of ~5% to the total known mare volume. If cryptomafic deposits are as common in other parts of the Moon as they are here, then volcanic deposits could make up as much as 42% of the surface area of the Moon and increase the total volume of extrusive deposits by as much as 76% over the current mare estimates. Our preliminary analyses suggest that it is unlikely that this study region is typical of the rest of the Moon, thus other regions are not expected to contain such large amounts of cryptomafic materials. Therefore, these extrapolations provide an upper estimate of the total lunar cryptomafic complement. Documentation of the cryptomafic deposits in this study region and elsewhere on the Moon will provide significant information on the location, flux, and onset of lunar volcanism. This information has major significance for the derivation and testing of models on lunar evolution and the role of volcanism on the Moon.

Acknowledgments: We gratefully acknowledge NASA Grant NAGW-2188 to JWH from the Planetary Geology and Geophysics Program. Special thanks are extended to Aileen Yingst for helpful and insightful discussions, to Paul Haggerty for unparalleled technical support, and to the University of Toronto, Department of Geology for the generous loan of their computer facilities.



References

Adams, J.B., and T.B. McCord, Vitrification darkening in the lunar highland and identification of Decartes material at the Apollo 16 site, Proc. 4th Lunar Sci. Conf., 163-177, 1973.

Antonenko, I., J.W. Head, J.F. Mustard, and B.R. Hawke, Criteria for the detection of lunar cryptomaria, Earth, Moon, Planets, 69, 141-172, 1995.

Basaltic Volcanism Study Project, Basaltic Volcanism on the Terrestrial Planets, 1286 pp, Pergamon, New York, NY, 1981.

Bell, J.F., and B.R. Hawke, Lunar Dark-haloed impact craters: Origins and implications for early mare volcanism, J. Geophys. Res., 89, 6899-6910, 1984.

Blewett, D.T., B.R. Hawke, P.G. Lucey, G.J. Taylor, R. Jaumann, and P.D. Spudis, Remote sensing and geologic studies of the Schiller-Schickard region of the Moon, J. Geophys. Res., 100, 16,959-16,978, 1995.

Bowker, D.E., and J.K. Hughes, Lunar Orbiter Photographic Atlas of the Moon, NASA, Washington, D.C., 1971.

Croft, S.K., The scaling of complex craters, Proc. Lunar Planet. Sci. Conf., 15th, Part 2, J. Geophys. Res., 90, C828-842, 1985.

De Hon, R.A., Thickness of the western mare basalts, Proc. Lunar Planet. Sci. Conf., 10th, 2935-2955, 1979.

Greeley, R., S.D. Kadel, D.A. Williams, L.R. Gaddis, J.W. Head, A.S. McEwen, S.L. Murchie, E. Nagel, G. Neukum, C.M. Pieters, J.M. Sunshine, R. Wagner, and M.J.S. Belton, Galileo Imaging Observations of Lunar Maria and Related Deposits, J. Geophys. Res. 98, 17,183-17,205, 1993.

Hapke, B., Theory of Reflectance and Emittance Spectroscopy, 455pp, Cambridge University Press, New York, NY, 1993.

Hawke, B.R., and J.F. Bell, Remote sensing studies of lunar dark-halo impact craters: Preliminary results and implications for early volcanism, Proc. Lunar Planet. Sci. Conf., 12th, 665-678, 1981.

Hawke, B.R. and J.W. Head, Impact melt on lunar crater rims, in Impact and Explosion Cratering, D.J. Roddy, R.O. Pepin, and R.B. Merrill (eds.), Pergamon, New York, N.Y. 815-841,1977.

Hawke, B.R. and P.D. Spudis, Geochemical anomalies on the eastern limb and farside of the Moon, in Proc. Conf. Lunar Highlands Crust, J.J. Papike and R.B. Merrill (eds.), Pergamon, New York, N.Y., 467-481, 1980.

Hawke, B.R., C.A. Peterson, P.G. Lucey, G.J. Taylor, D.T. Blewett, B.A. Campbell, C.R. Coombs, and P.D. Spudis, Remote sensing studies of the terrain northwest of Humorum basin, Geophys. Res. Lett., 20, 419-422, 1993.

Head, J.W., Lunar mare deposits: Areas, volumes, sequence, and implication for melting in source areas, in Origins of Mare Basalts and their Implications for Lunar Evolution, Lunar Science Institute, Houston, TX, 66-69, 1975a.

Head, J.W., Mode of occurrence and style of emplacement of lunar mare deposits, in Origins of Mare Basalts and their Implications for Lunar Evolution, Lunar Science Institute, Houston, TX, 61-65, 1975b.

Head, J.W., Lava flooding of ancient planetary crusts: Geometry, thickness, and volumes of flooded lunar impact basins, The Moon and the Planets, 26, 61-88, 1982.

Head, J.W., and L. Wilson, Lunar mare volcanism: Stratigraphy, eruption conditions, and the evolution of secondary crusts, Geochim. et Cosmochim. Acta, 56, 2144-2175, 1992.

Head, J.W., S.M. Murchie, J.F. Mustard, C.M. Pieters, G. Neukum, A.S. McEwen, R.F. Greeley, E. Nagel, and M.J.S. Belton, Lunar impact basins: New data for the western limb and far side (Orientale and South Pole-Aitken basins) from the first Galileo flyby, J. Geophys. Res., 98, 17,149-17,181, 1993.

Heiken, G.H., D.T. Vaniman, B.M. French, Lunar Sourcebook: A User's Guide to the Moon, Cambridge University Press, New York, NY, p736, 1991.

Hess, P.C., and E.M. Parmentier, A model for the thermal and chemical evolution of the Moon's interior: Implications for the onset of mare volcanism, Earth Planet. Sci. Letts., 134, 501-514, 1995.

Karlstrom, T.N.V., Geologic map of the Schickard quadrangle of the Moon, scale 1:1,000,000, Map I-823, USGS, Department of the Interior, Washington, DC, 1974.

Kuiper, G.P., E.A. Whitaker, R.G. Strom, J.W. Fountain, and S.M. Larson, Consolidated Lunar Atlas, University of Arizona Press, Tucson, AZ, 1967.

Longhi, J., Experimental petrology and petrogenesis of mare volcanics, Geochim. et Cosmochim. Acta, 56, 2235-2252, 1992.

Lucey, P.G., B.C. Bruno, and B.R. Hawke, Preliminary results of imaging spectroscopy of the Humorum basin region of the Moon, Proc. Lunar Planet. Sci. Conf., 21, 391-403, 1991.

Marshall, C.H., Geologic map and sections of the Letronne region of the Moon, scale 1:1,000,000, Map I-385, USGS, Department of the Interior, Washington, DC, 1963.

McCauley, J.F., Geologic map of the Hevelius region of the Moon, scale 1:1,000,000, Map I-491, USGS, Department of the Interior, Washington, DC, 1967.

McCauley, J.F., Geologic map of the Grimaldi quadrangle of the Moon, scale 1:1,000,000, Map I-740, USGS, Department of the Interior, Washington, DC, 1973.

McGetchin, T.R., M. Settle, and J.W. Head, Radial thickness variation in impact crater ejecta: implications for lunar basin deposits, Earth Planet. Sci. Letters, 20, 226-236, 1973.

Melosh, H.J., Impact Cratering: A Geologic Process, 245pp, Oxford University Press, New York, NY, 1989.

Mustard, J.F., and J.W. Head, Buried stratigraphic relationships along the southwestern shores of Oceanus Procellarum: Implications for early lunar volcanism, J. Geophys. Res., 101, 18,913-18,925, 1996.

Mustard, J.F., J.W. Head, and I. Antonenko, Mare-highland mixing relationships along the southwestern shores of Oceanus Procellarum, Lunar and Planet. Sci. Conf. , 24, 963-964, 1994.

Mustard, J.F., J.W. Head, S.M. Murchie, C.M. Pieters, M.S. Belton, and A.S. McEwen, Schickard Cryptomare: Interaction between Orientale ejecta and pre-basin mare from spectral mixture analysis of Galileo SSI data, Lunar and Planet. Sci. Conf. , 23, 957-958, 1992.

Neal, C.R., and L.A. Taylor, Petrogenesis of mare basalts: A record of lunar volcanism, Geochim. et Cosmochim. Acta, 56, 2177-2212, 1992.

Nyquist, L.W. and C.-Y. Shih, The Isotopic Record of Lunar Volcanism, Geochim. et Cosmochim. Acta 56, 2213-2234, 1992.

Oberbeck, V.R., The role of ballistic erosion and sedimentation in lunar stratigraphy, Rev. Geophys. Space Phys. , 13, 337-362, 1975.

Offield, T.W., Geologic map of the Schiller quadrangle of the Moon, scale 1:1,000,000, Map I-691, USGS, Department of the Interior, Washington, DC, 1971.

Pieters, C.M., J.W. Head, J.M. Sunshine, E.M. Fischer, S.L. Murchie, M.J.S. Belton, A.S. McEwen, L.R. Gaddis, R. Greeley, G. Neukum, R. Jaumann, and H. Hoffmann, Crustal diversity of the Moon: Compositional analyses of Galileo Solid State Imaging data, J. Geophys. Res., 98, 17,127-17,148, 1993.

Pike, R.J., Control of crater morphology by gravity and target type: Mars, Earth, Moon, Proc. Lunar Planet. Sci. Conf., 11th, 2159-2189, 1980.

Saunders, R.S., and D.E. Wilhelms, Geologic map of the Wilhelm quadrangle of the Moon, scale 1:1,000,000, Map I-824, USGS, Department of the Interior, Washington, DC, 1974.

Schultz, P.H., and P.D. Spudis, Evidence for ancient mare volcanism, Proc. Lunar Planet. Sci. Conf., 10th, 2899-2918, 1979.

Schultz, P.H., and P.D. Spudis, Beginning and end of lunar mare volcanism, Nature, 302, 233-236, 1983.

Schultz, P.H., D. Orphal, B. Miller, W.F. Borden, and S.A. Larson, Multi-ring basin formation: Possible clues from impact cratering calculations, in Multi-ring Basins, Proc. Lunar Planet. Sci., 12A, P.H. Schultz and R.B. Merrill (eds.), 181-195, 1981.

Scott, D.H., J.F. McCauley, and M.N. West, Geologic map of the west side of the Moon, scale 1:5,000,000, Map I-1034, USGS, Department of the Interior, Washington, DC, 1977.

Solomon, S.C., and J.W. Head, Vertical movement in mare basins: Relation to mare emplacement, basin tectonics, and lunar thermal history, J. Geophys. Res., 84, 1667-1682, 1979.

Solomon, S.C., and J.W. Head, Lunar mascon basins: Lava filling, tectonics, and evolution of the lithosphere, Rev. Geophys. and Space Phys., 18, 107-141, 1980.

Solomon, S.C., R.J. Ahrens, P.M. Cassen, A.T. Hsui, J.W. Minear, R.T. Reynolds, N.H. Sleep, D.E. Strangway, and D.L. Turcotte, Thermal histories of the terrestrial planets, Basaltic Volcanism on the Terrestrial Planets, Chap. 9, Pergamon, NY, 1129-1234, 1981.

Stöffler, D., D.E. Gault, J. Wedekind, and G. Polkowski, Experimental hypervelocity impact into quartz sand: Distribution and shock metamorphism of ejecta, J. Geophys. Res., 80, 4062-4077, 1975.

Taylor, L.A., J.W. Shervais, R.H. Hunter, and J.C. Laul, Ancient (4.2 AE) highlands volcanism: The gabbronorite connection? (abstract), Lunar Planet. Sci., XIV, 777-778, 1983.

Taylor, S.R., Growth of planetary crusts, Tectonophysics, 161, 147-156, 1989.

Titley, S.R., Geologic map of the Schickard quadrangle of the Moon, scale 1:1,000,000, Map I-495, USGS, Department of the Interior, Washington, DC, 1967.

Whitaker, E.A., G.P. Kuiper, W.K. Hartmann, and L.H. Spradley, Rectified Lunar Atlas, University of Arizona Press, Tucson, AZ, 1963.

Wilhelms, D.E., The geologic history of the Moon, USGS Professional Paper 1348, United States Government Printing Office, Washington, DC, 1987.

Wilhelms, D.E., K.A. Howard, and H.G. Wilshire, Geologic map of the south side of the Moon, scale 1:5,000,000, Map I-1162, USGS, Department of the Interior, Washington, DC, 1979.

Wilshire, H.G., Geologic map of the Byrgius quadrangle of the Moon, scale 1:1,000,000, Map I-755, USGS, Department of the Interior, Washington, DC, 1973.

Yingst, R. A., and J.W. Head, Lunar mare deposit volumes, composition, age, and location: Implications for source areas and modes of emplacement, Lunar and Planet. Sci. Conf. , 24, 1531-1532, 1994.

Yingst, R. A., and J.W. Head, Volumes of lunar lava ponds in South Pole-Aitken and Orientale basins: Implications for eruption conditions, transport mechanisms and magma source regions, J. Geophys. Res.- Planets, 102, 10,909-10,931, 1997.

Zuber, M.T., D.E. Smith, F.G. Lemoine, and G.A. Neumann, The shape and internal structure of the Moon from the Clementine mission, Science, 266, 1839-1843, 1994.



Tables

Table 1. Listing of dark-haloed craters found in the Schiller-Schickard area of the Moon. Latitudes and longitudes were obtained from lunar maps. Distance from Orientale was calculated from the center of Orientale basin (19.3 ºS, 94.2 ºW), using the latitude and longitude co-ordinates, and spherical geometry. Crater diameters were measured from rim crest to rim crest of each crater, using the Lunar Orbiter Photographic Atlas of the Moon [Bowker and Hughes, 1971]. The excavation depth (d_e) was calculated using the relation d_e ~ 0.084 D_r for simple craters and d_e ~ 0.15 D_r^0.85 for complex craters, as discussed in the text. An error of ~15% is associated with the values derived using these equations.

36.9 ºS	26.0 ºW	1862	4.3	361
37.4 ºS	25.5 ºW	1874	6.8	571
39.0 ºS	55.6 ºW	1172	4.8	403
39.0 ºS	57.9 ºW	1121	4.8	403
39.8 ºS	31.5 ºW	1730	4.4	370
40.0 ºS	50.5 ºW	1295	6.0	504
40.3 ºS	57.1 ºW	1153	5.2	437
40.7 ºS	58.6 ºW	1126	14.4	1210
40.8 ºS	42.1 ºW	1485	3.4	286
41.3 ºS	52.4 ºW	1265	9.0	756
42.7 ºS	44.6 ºW	1445	14.6	1226
42.9 ºS	29.6 ºW	1776	15.5	1302
43.3 ºS	36.6 ºW	1623	10.3	865
44.0 ºS	53.2 ºW	1277	4.7	395
44.1 ºS	53.8 ºW	1266	5.6	470
44.3 ºS	53.6 ºW	1273	2.9	244
44.4 ºS	67.3 ºW	1020	7.3	613
44.7 ºS	41.2 ºW	1532	2.7	227
44.8 ºS	25.5 ºW	1868	12.2	1025
45.2 ºS	28.6 ºW	1801	13.4	1126
45.8 ºS	39.1 ºW	1582	4.3	361
45.8 ºS	55.8 ºW	1249	13.3	1117
46.3 ºS	35.8 ºW	1653	3.6	302
46.4 ºS	46.4 ºW	1439	3.9	328
46.6 ºS	25.6 ºW	1866	6.6	554
47.0 ºS	59.0 ºW	1209	21.4	2027
48.0 ºS	46.6 ºW	1451	9.4	790
48.6 ºS	40.7 ºW	1570	9.8	823
49.3 ºS	51.0 ºW	1384	4.8	403
49.9 ºS	67.7 ºW	1127	10.9	916
50.0 ºS	50.7 ºW	1399	8.5	714
50.0 ºS	63.4 ºW	1191	5.5	462
50.1 ºS	59.2 ºW	1257	2.2	185
50.3 ºS	60.2 ºW	1245	3.8	319
50.7 ºS	62.2 ºW	1222	4.9	412
51.3 ºS	49.0 ºW	1445	8.2	689
52.1 ºS	51.9 ºW	1408	2.1	176
52.7 ºS	26.0 ºW	1869	6.8	571
52.7 ºS	47.0 ºW	1497	5.7	479
53.7 ºS	57.4 ºW	1349	3.9	328
54.1 ºS	46.9 ºW	1516	3.6	302
54.2 ºS	60.6 ºW	1313	6.5	546
55.4 ºS	34.7 ºW	1730	2.2	185
55.4 ºS	38.0 ºW	1676	2.9	244

Table 2. Listing of dark-haloed craters found in the region west of Mare Humorum. Latitudes and longitudes were obtained from lunar maps. Distance from Orientale was calculated from the center of Orientale basin (19.3 ºS, 94.2 ºW), using the latitude and longitude co-ordinates, and spherical geometry. Crater diameters were measured from rim crest to rim crest of each crater, using the Lunar Orbiter Photographic Atlas of the Moon [Bowker and Hughes, 1971]. The excavation depth (d_e) was calculated using the relation d_e ~ 0.084 D_r for simple craters and d_e ~ 0.15 D_r^0.85 for complex craters, as discussed in the text. An error of ~15% is associated with the values derived using these equations.

Latitude	Longitude	Distancefrom Orientale (km)	Diameter Dr (km)	Excavation Depth de (m)
13.2 ºS	58.4 ºW	1057	5.4	454
13.3 ºS	57.3 ºW	1088	6.6	554
14.0 ºS	46.2 ºW	1400	7.0	558
14.2 ºS	47.5 ºW	1361	3.0	252
14.4 ºS	43.6 ºW	1472	4.8	403
14.8 ºS	41.5 ºW	1529	2.5	210
14.9 ºS	45.0 ºW	1428	8.1	680
15.4 ºS	59.9 ºW	999	2.6	218
16.7 ºS	44.5 ºW	1431	7.8	655
16.7 ºS	61.0 ºW	960	3.6	302
16.8 ºS	43.4 ºW	1462	3.1	260
16.9 ºS	62.0 ºW	930	11.5	966
17.2 ºS	58.5 ºW	1029	6.8	571
17.8 ºS	48.0 ºW	1325	3.4	286
18.3 ºS	67.9 ºW	755	5.3	445
18.5 ºS	68.5 ºW	737	4.3	361
18.9 ºS	61.1 ºW	947	2.9	244
19.5 ºS	63.3 ºW	883	9.5	798
20.0 ºS	58.5 ºW	1018	7.6	638
20.4 ºS	56.5 ºW	1071	6.5	546
20.9 ºS	52.8 ºW	1177	5.9	496
21.0 ºS	51.6 ºW	1211	13.5	1134
21.7 ºS	53.0 ºW	1170	3.1	260
21.7 ºS	53.7 ºW	1150	5.3	445
22.0 ºS	69.8 ºW	697	4.2	353
22.5 ºS	46.0 ºW	1364	10.0	840
22.8 ºS	69.5 ºW	706	8.4	706
23.7 ºS	52.4 ºW	1183	4.8	403
24.1 ºS	51.6 ºW	1205	4.3	361
24.3 ºS	60.4 ºW	962	3.8	319
24.5 ºS	60.0 ºW	973	4.4	370
25.3 ºS	58.7 ºW	1010	2.4	202
25.5 ºS	56.5 ºW	1070	2.3	193
26.0 ºS	67.7 ºW	768	9.7	815
26.2 ºS	51.7 ºW	1202	6.2	521
26.7 ºS	57.9 ºW	1034	5.4	454
27.9 ºS	50.5 ºW	1236	2.6	218
28.8 ºS	55.9 ºW	1094	5.5	462
29.2 ºS	56.5 ºW	1080	3.1	260
29.5 ºS	49.1 ºW	1275	4.4	370
29.9 ºS	64.7 ºW	872	21.3	2019
30.1 ºS	52.5 ºW	1188	4.8	403
30.2 ºS	52.9 ºW	1178	3.7	311
30.4 ºS	48.2 ºW	1301	6.1	512
33.4 ºS	44.5 ºW	1403	5.2	437
33.6 ºS	49.2 ºW	1285	10.7	899
34.0 ºS	49.6 ºW	1277	5.1	428
34.7 ºS	49.4 ºW	1285	1.8	151
35.4 ºS	37.7 ºW	1576	3.2	269
35.8 ºS	44.0 ºW	1423	3.1	260

Table 3. Listing of dark-haloed craters found in the area west of Oceanus Procellarum. Latitudes and longitudes were obtained from lunar maps. Distance from Orientale was calculated from the center of Orientale basin (19.3 ºS, 94.2 ºW), using the latitude and longitude co-ordinates, and spherical geometry. Crater diameters were measured from rim crest to rim crest of each crater, using the Lunar Orbiter Photographic Atlas of the Moon [Bowker and Hughes, 1971]. The excavation depth (d_e) was calculated using the relation d_e ~ 0.084 D_r for simple craters and d_e ~ 0.15 D_r^0.85 for complex craters, as discussed in the text. An error of ~15% is associated with the values derived using these equations.

Latitude	Longitude	Distancefrom Orientale (km)	Diameter Dr (km)	Excavation Depth de (m)
12.4 ºN	80.0 ºW	1051	21.8	2060
11.4 ºN	77.8 ºW	1052	6.9	580
11.2 ºN	78.8 ºW	1030	12.5	1050
8.7 ºN	90.1 ºW	854	10.2	857
5.8 ºN	69.2 ºW	1066	8.5	714
3.9 ºN	73.5 ºW	936	13.6	1142
1.8 ºN	75.2 ºW	854	2.4	202
0.5 ºS	68.8 ºW	946	7.5	630
1.5 ºS	71.6 ºW	861	13.9	1168

Figures


Figure 1. a) Zond 8 image showing the western limb of the Moon at high sun-angle. A good example of a dark-haloed crater, indicated by the white arrow, can be seen in this image. This dark-haloed crater was found in the Oceanus Procellarum study area.




Figure 1. b) Sketch map of the region shown in Figure 1a. The location of the three study areas is indicated by boxes: Schiller-Schickard lower-most, area west of Mare Humorum center, and the area west of Oceanus Procellarum upper-most box.




Figure 2. Diagrammatic representation of the formation of a cryptomafic deposit by the emplacement of a high albedo basin ejecta deposit on top of a pre-existing low albedo mafic deposit. These hidden mafic units may be sampled by impact craters that penetrate the regional ejecta to excavate mafic material, emplacing it in a symmetrical dark halo around the crater. Craters that are too small to penetrate to the mafic substrate will not form dark halos. If volcanism continues to occur after the emplacement of the regional ejecta unit, and the formation of the cryptomafic deposit, post-basin mafic patches will be formed. Modified from Antonenko et al. [1995].




Figure 3. Schematic diagram illustrating how the excavation depth of the smallest dark-haloed craters (Min DHC's) defines the top of the cryptomafic deposit, and thus the thickness of the overlying ejecta, while the excavation depth of the largest dark-haloed craters (Max DHC's) defines the bottom of the cryptomafic deposit. Note how the size of the smallest dark-haloed craters varies as a function of distance from the ejecta source; minimum craters closer to the ejecta source are larger since they must penetrate through thicker, proximal deposits. Very small craters will not form dark-haloed craters, because they do not penetrate to the mafic layer. Very large craters will also not form dark-haloed craters, because they excavate enough highland crustal material to obscure the dark halo signature.




Figure 4. Sketch map showing the location of dark-haloed craters on the western limb of the Moon. The three study areas are located in the lower right (Schiller-Schickard), center right (west of Mare Humorum), and in the upper center (west of Oceanus Procellarum). Crater locations are listed in Tables 1, 2, and 3, respectively.




Figure 5. a) Image of Schickard crater area from the Consolidated Lunar Atlas [Kuiper et al., 1967] showing examples of dark-haloed craters (black arrows) and craters (white arrows) that were eliminated from consideration due to halo asymmetry, sparseness, or potential contamination from nearby dark-haloed craters.




Figure 5. b) Image of two non-dark-haloed craters found west of Schiller crater. The non-dark-haloed craters, indicated by white arrows, are both approximately 6 km in diameter. They are located on a slight topographic rise, possibly related to the rim of Schiller crater, around which the cryptomafic volcanics appear to have flowed. This image is taken from the data set of Lunar Orbiter IV, Frame 160 H1.




Figure 6. Block diagram of the smallest possible dark-haloed crater that can form (Min DHC), where the dark halo is just barely begun to be visible, and the largest possible dark-haloed crater that can form (Max DHC), where the dark halo is still barely visible despite the presence of obscuring highland material. The depth of excavation (d_e) is know to be one third of the crater depth (d_r) [Stöffler, 1975]. For the minimum dark-haloed crater, the relationship between depth of excavation (d_e), thickness of the overlying regional ejecta layer (t_m), and penetration into mafic material (d_m) that is required to produce a barely visible dark halo, is shown to be d_m = d_e - t_m. For the maximum dark-haloed crater, the relationship between depth of excavation (d_e), thickness of the combined overlying layers (t_h), and penetration into highland crustal material (d_h) that can occur before sufficient highland material is excavated for the dark halo to be obscured, is shown to be d_h = d_e - t_h. More highland material is required to obscure a dark halo than mafic material is required to produce a dark halo, thus d_h > d_m.




Figure 7. Sketch map of the Schiller-Schickard cryptomafic region (speckled area), showing the distribution of dark-haloed craters (black circles; Table 1) and non-dark-haloed craters (open circles). The boundary (thin solid line) of the cryptomafic deposit is defined by the distribution of dark-haloed craters and non-dark-haloed craters, as discussed in the text. Portions of the boundary, where question marks are shown, are poorly defined due to the absence of good, high sun-angle image data at these locations. The solid black areas indicate post-Orientale mare patches.




Figure 8. Thickness estimates for the layer overlying the Schiller-Schickard cryptomafic deposit plotted as a function of the distance from Orientale. Column heights represent the estimated thicknesses, as determined from the depths of excavation of the smallest dark-haloed craters in each 100-km distance interval. Numbers at the base of each column represent the number of craters in each bin. Also shown are theoretical decay curves for the thickness of Orientale ejecta [McGetchin et al., 1973; Schultz et al., 1981], calculated for a 900 km and a 620 km Orientale diameter [Wilhelms, 1987].




Figure 9. Thickness estimates for the cryptomafic deposit in Schiller-Schickard area, plotted as a function of distance from Orientale basin. Column heights represent the estimated thicknesses, calculated from the difference between excavation depths of the largest and smallest dark-haloed craters in each 100-km distance interval. Numbers at the base of each column represent the number of craters in each bin. The first distance interval is shown empty because only one dark-haloed crater was located within this interval. The average thickness of the cryptomafic deposit is found to be approximately 900 meters.




Figure 10. Sketch map of the cryptomafic deposits (speckled areas) west of Mare Humorum, showing the distribution of dark-haloed craters (black circles; Table 2) and non-dark-haloed craters (open circles). Boundaries (thin solid lines) of the cryptomafic units are defined by the distribution of dark-haloed craters and non-dark-haloed craters, with the aid of spectral mixing and topographic cues as discussed in the text. Question marks are used to indicate uncertainty in the boundaries due to the absence of indicative craters, spectral evidence and topographic cues. The rim of Humorum basin, as defined by Wilhelms [1987], is shown using a dashed line. The solid black areas indicate post-Orientale mare patches




Figure 11. Thickness estimates for the layer overlying the western Mare Humorum cryptomafic deposit plotted as a function of the distance from Orientale. Column heights represent the estimated thicknesses, as determined from the excavation depths of the smallest dark-haloed craters in each 100-km distance interval. Numbers at the base of each column represent the number of craters in each bin. Also shown are the theoretical decay curves for the thickness of Orientale ejecta [McGetchin et al., 1973; Schultz et al., 1981], calculated for a 900 km and a 620 km Orientale diameter [Wilhelms, 1987]. The layer here appears to be relatively thinner than in the Schiller-Schickard region.




Figure 12. Thickness estimates for cryptomafic deposits in the area west of Mare Humorum, plotted as a function of the distance from Orientale. Column heights represent the estimated thicknesses, calculated from the difference between excavation depths of the largest and smallest dark-haloed craters in each 100-km distance interval. Numbers at the base of each column represent the number of craters in each bin. The first distance interval is shown empty because only one dark-haloed crater was located within this interval. The average thickness of the cryptomafic deposits here is found to be approximately 600 meters.




Figure 13. Stratigraphic column, which illustrates the sequence of geologic events in southwestern Oceanus Procellarum, as proposed by Mustard and Head [1996]. It is suspected that a considerable volume of basalt was emplaced here before the Orientale impact event. Locally, these basalt deposits may be interrupted by highland ejecta units, which were emplaced on top of the mare material by large craters, such as Letronne. In some areas, parts of the column may be missing from the geologic record altogether, where a basin impact event or the ballistic emplacement of basin ejecta has obliterated the pre-existing basalts. [From Mustard and Head, 1996].




Figure 14. Sketch map of the cryptomafic patches (speckled areas) west of Oceanus Procellarum, showing the distribution of dark-haloed craters (black circles; Table 3) and non-dark-haloed craters (open circles). The boundaries (thin solid lines) of the cryptomafic patches are defined by the distribution of dark-haloed craters and non-dark-haloed craters, aided by topographic cues and the distribution of light plains units as discussed in the text. Solid black areas indicate post-Orientale mare patches. The rims of old, degraded craters are identified by dashed lines.




Figure 15. Diagram illustrating the various modes of emplacement for mare deposits. The shape of the final mare deposit depends on the pre-fill topography and on the degree of filling that occurred. Mare names indicate examples of each of the various types. [From Head, 1975b].

Go to Chapter 2
Return to Chapter 1 - Table of Contents
               Thesis Title Page