2.3.1 Global Distribution of Stratified Deposits
Of the 17,073 HiRISE images examined in this study, 5,324 contain stratified rock in one or more geomorphic setting or deposit type (Figure 2.8). Counting images that have been double book-kept for the presence of stratified deposits in one or more settings, there are 5,781 unique stratified deposits observed in the HiRISE image inventory. Figures 2.8c and 2.8f show that stratified deposits occur ubiquitously throughout the latitude range from 60º N to 60º S, and are widespread over all longitudes. The histogram in Figure 2.8f shows a slight overall decrease in the number of images containing stratified deposits moving from the northern to southern hemispheres, but besides this weak trend there appears to be no clear latitudinal control on the distribution of stratified deposits that could not also be partially explained by the overall image distribution (Figure 2.8e), as described below.
Several regions show a particularly high density of images containing stratified deposits, including Valles Marineris, Meridiani Planum, Hellas basin, Aeolis Mensae, and the fretted terrains of Deuteronilus, Protonilus, Nilo Syrtis Mensae. Although stratified deposits are common and widespread in these regions, these areas of high concentration (Figure 2.8d) also coincide with the areas of highest image density (Figure 2.8b), suggesting that an image acquisition bias may be partly responsible for the density of images in this area. Histograms also show that latitude ranges containing a large number of images with stratified deposits also coincide with latitudes where the number of overall images taken is high. This is the case for the 0 to 20º S range, which includes the high density areas of Valles Marineris, Meridiani Planum, and Aeolis Mensae, as well as the 30-
21 40º S range, which includes images of stratified material in southern Terra Cimmeria and Terra Sirenum. In contrast, there is a disproportionate number of images in the 30-45º N range that contain stratified deposits relative to the overall number of images taken in this region. This suggests that the high concentration of images containing stratified deposits in the Deuteronilus, Protonilus, and Nilo Syrtis Mensae regions truly represents an abundance of stratified deposits present in this area that is not simply a reflection of an image acquisition bias in this area. Areas with a relatively low density of images containing stratified deposits include northern Terra Cimmeria and eastern Tyrrhena Terra, Terra Sabaea, and northern Noachis Terra.
2.3.2 Geomorphic Setting of Stratified Deposits
Crater interiors are the most common and widespread settings in which stratified deposits are found on Mars (Figure 2.9b), and images containing stratified rocks in crater interiors occur at all latitude ranges. Plains deposits are the second most common setting for stratified deposits. These deposits are most prevalent in the northern hemisphere, and are concentrated around Valles Marineris, Meridiani Planum and western Arabia Terra, and near the dichotomy boundary in the fretted terrains of Dueteronilus, Protonilus, and Nilo Syrtis Mensae, Nili Fossae, and Aeolis Mensae. These fretted terrains contain an abundance of buttes and mesas; where these mesas form an interconnected network of valleys, they and any deposits between the mesas were identified as walls or fills in the canyon/chasm category, respectively. Where these deposits were isolated, and not clearly part of an interconnected network of mesas and buttes, they were identified as plains
22 deposits. Stratified plains deposits also occur throughout Elysium Planitia and the Tharsis region.
Canyon deposits are largely concentrated near the equator in Valles Marineris and in the northern hemisphere, particularly along the dichotomy boundary between the southern highlands and northern lowlands. Aside from areas of high density, canyon deposits are also observed to be fairly widespread throughout the Elysium Planitia and Tharsis regions, where layered lavas flows are frequently exposed in fissure and catena walls, both landforms tabulated in the canyon/chasm category. Aside from deposits north of Argyre Planitia, in the Uzboi-Margaritifer-Ladon system, and Niger and Dao Vallis in eastern Hellas Basin, canyon/chasm deposits are sparse below ~15º S. The “other” category of deposits, which includes volcanic constructs, crater ejecta, and odd features like the Acidalia mounds, are also largely limited to the northern lowlands.
As seen in Figure 2.9c, most of the stratified deposits found in craters are located in Noachian-aged terrains. This is not unexpected, as the oldest terrains on Mars are also the most heavily cratered. Accordingly, the percent of images containing stratified deposits in craters decreases systematically from Noachian (45%) to Hesperian (33%) to Amazonian- aged terrains (33%) (Figure 2.12). Canyon, chasm, and channel deposits are most often found in Hesperian-aged terrains (Figure 2.9c), and are concentrated around the Valles Marineris canyon system. Few images containing canyon or channel deposits are found in Noachian-aged terrains, consistent with the paucity of these deposits in the southern highlands of Mars. Plains deposits are found most commonly in the layered lavas that compose Amazonian-aged terrains, e.g., Tharsis region and Elysium Planitia, but also in
23 Noachian-aged terrains such as Meridiani Planum and in the plains north of Hellas basin.
Very few of the “other” category of deposits are found in Noachian-age terrains, instead occurring nearly equally in terrains of Hesperian of Amazonian age. Again, this is not unexpected, as this category consists largely of volcanic constructs of Hesperian or Amazonian age and deposits associated with well-defined crater ejecta at the modern surface, where the latter is not likely to be preserved due to erosion for older Noachian craters.
2.3.3 Basin Fill versus Unconfined Stratified Deposits
The global distribution of basin fill deposits (fills and mounds) versus unconfined stratified deposits (wall, uplift, plains, and other deposits) is presented in Figures 2.10 and 2.11. In total, unconfined stratified deposits outnumber basin fill deposits (Figure 2.12b) and are widespread across the surface of Mars in terrains of all ages (Figure 2.10a and Figure 2.11a-2.11c). In contrast, basin fill deposits occur predominantly in the southern highlands in terrains of Noachian and Hesperian age, where they are fairly widespread (Figure 2.10b). In Amazonian-aged terrains, basin-fill deposits are localized to a few locations, including Valles Marineris, Dueteronilus, Protonilus, and Nilo Syrtis Mensae, and Elysium and Utopia Planitia. The plots in Figure 2.12 illustrate the changes in the global distribution and relative proportions of basin fill and unconfined stratified deposits in terrains of different age. The data exhibit an overall decrease in the percentage of basin fill deposits found in successively younger terrains (Figure 12a); 42% of all basin fill deposits are located in Noachian-aged terrains, 36% are found in Hesperian-aged regions,
24 and only 22% are found in Amazonian-aged terrains. In contrast, the percentage of unconfined stratified deposits, which consists predominantly of crater, canyon, and channel wall deposits, found in Noachian (30%), Hesperian (35%), and Amazonian (35%) terrains remains fairly constant (Figure 2.12a). The successive decrease in the proportion of basin fill deposits in younger terrains is also illustrated in Figure 2.12b, in which Noachian-aged terrains contain 40% basin fill deposits and 60% unconfined stratified deposits, while Amazonian terrains contain only 23% basin fill deposits and 77% unconfined stratified deposits.
2.3.4 Glacial/Periglacial Deposits
The distribution of all stratified deposits associated with dissected mantle terrain or viscous flow features, including concentric crater fills, lobate debris apron, or lineated valley fill, are presented in Figure 2.13. There is a clear latitudinal control on these deposits as they fall strictly between 30-60º bands N or S of the equator, with those deposits in the north outnumbering those in the south. The northern glacial/periglacial deposits occur in the northern lowlands and in high concentration at the dichotomy boundary within Dueteronilus and Nilo Syrtis Mensae. The southern deposits are sparsely distributed, with the only major concentration occurring within Hellas basin (Figure 2.6b).
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