2.4 Discussion
2.4.3 Global-Scale Depositional Processes on Mars
30 widespread lithification and cementation of basin-filling sedimentary deposits, i.e., Andrews-Hanna et al. [2010]. The transition to arid conditions during the Late Noachian [Andrews-Hanna and Lewis, 2011] associated with the loss of water and a deepening of the water table would have prevented the widespread preservation of stratified sedimentary deposits in younger terrains on Mars even if the sedimentary cycle had remained active into the Amazonian.
31 depositional processes operating on planet-wide scales. The location of major volcanic centers relative to the global distribution of stratified deposits (Figure 2.14) suggests that many of the stratified deposits in Amazonian-aged terrains, particularly those in the vicinity of these volcanic centers, can be explained by effusive lava flows. Examples of stratified lavas can be found in the global database, particularly in the layered scarps and flanks of the major Martian volcanoes, stratified lobes of lavas in plains in the Tharsis region and Elysium Planitia, and in the stratigraphy preserved in the walls of catenas, pit craters, and fissures in close proximity to these major volcanic centers (Figure 2.14). These findings are consistent with those of Bandfield et al. [2013], who find a predominance of blocky effusive lavas in younger, Amazonian-aged terrains. Pyroclastic volcanism, as proposed by Kerber et al. [2012] and Bandfield et al. [2013], may be more effective at widespread distribution of material over the surface of Mars. Isopach maps of predicted pyroclastic deposit thickness produced by Kerber et al. [2012] plotted together with this study’s stratified deposit database (Figure 2.14) show that many stratified deposits fit within the predicted regions of thick pyroclastic deposition. Approximately 3000 stratified deposits are located in regions near major volcanic centers and in areas of predicted thick pyroclastic deposition (Figure 2.14), suggesting that ~50% of all observed stratified deposits could be reasonably explained by extrusive volcanic processes. However, as Kerber et al. noted and can be seen in Figure 2.14, models of pyroclastic accumulation and proximity to volcanic regions cannot fully explain the distribution of stratified deposits, including some of the most widespread and conspicuous deposits in Arabia Terra and Meridiani Planum.
32 Glacial/periglacial processes can partially explain the distribution of stratified deposits, particularly at latitude ranges greater than 30º N and S, where these processes are known to occur [Milliken et al., 2003; Head et al., 2003; Schon et al., 2009].
Approximately 400 basin fill and intercrater plains deposits observed in the database are associated with glacial/periglacial features, but nearly half (43%) of the entire inventory of 5,777 stratified deposits is located between 30-60° N or S in regions known to host numerous glacier-like forms [Souness et al., 2012]. Of course not all of the deposits observed in these latitude ranges exhibit evidence for glacial processes, but the predominance of stratified deposits in these regions suggests that low temperature processes are important, and perhaps underappreciated, contributors to the stratified rock record of Mars.
Quantifying the relative importance of sedimentary processes to the Martian rock record is particularly challenging because unique criteria for the identification of sedimentary rocks in orbital images have not been established. Furthermore, predicting the global distribution of sedimentary rocks is made difficult by the variety of processes that can produce and transport sedimentary materials on both local and global scales. However, the database presented here can provide some initial quantitative constraints on the relative importance of sedimentary processes. Of the 5,781 total stratified deposits observed in the HiRISE database, 1,856 are basin fill deposits. Basin fill deposits need not be sedimentary in origin, but their occurrence in defined topographic basins suggests the transport and deposition of material. Furthermore, examples in the database where stratified basin fills are suspected to be layered lava flows are rare. Rather, most basin fills are similar to the
33 examples presented in Figure 2.2a-2.2h and Figure 2.6, and are most likely formed by sedimentary, glacial/periglacial, or volcaniclastic processes. In addition approximately half of all observed intercrater plains (~700 deposits) and wall/uplift deposits (~1000) are located in regions with no nearby volcanic centers, where predicted pyroclastic accumulation is fairly low (Figure 2.14). The basin fill deposits together with the intercrater plains and wall/uplift deposits not located near major volcanic centers make up ~50% of the deposits observed in this study, and represent an initial estimate for the contribution of sedimentary processes, although the ~400 deposits of glacial/periglacial origin are also included in this value. As mentioned above, in situ observations are likely needed to make more conclusive process-based distinctions. Still, the widespread occurrence of basin-fill deposits, particularly in the oldest terrains of Mars, where predicted pyroclastic accumulations are low and evidence for glacial/periglacial processes is sparse, requires the likely widespread occurrence of eolian, fluvial, and/or lacustrine sedimentary processes.
In summary, sedimentary and periglacial/glacial processes account for at least half of the stratified rock record of Mars. The other half of the deposits observed in the database (primarily crater and canyon wall deposits in the unconfined basin classification) could be reasonably explained by extrusive volcanic processes. Periglacial/glacial processes may be important contributors to the occurrence of stratified deposits observed at latitudes above 30º. The role of impact processes as a producer and transporter of sediment remains unquantified, and additional work is needed to understand the relative importance of impact processes in relation to the processes described above.
34 In the absence of widely applicable absolute age dating techniques on Mars, high- resolution image data sets and spectral observations become the primary tools for correlating spatially distinct deposits. However, on a largely basaltic planet such as Mars, mineral assemblages need not uniquely reflect one particular depositional process or time period, and may only be applicable for stratigraphic correlations in the most general sense, i.e., Bibring et al., [2006]. The global distribution of stratified deposits presented here can aid in identifying regions on Mars where orbital stratigraphic correlations may be most successful. For example, high-density areas of unconfined stratified deposits in terrains of similar age may allow successful correlations between spatially distinct crater and canyon wall deposits. Good candidates for this type of future analysis include Noachian-aged terrains in Terra Cimmeria, Terra Sirenum, Meridiani Planum, and Western Arabia Terra, Hesperian deposits in Valles Marineris, and Amazonian-aged terrains in Elysium, Utopia, and Deuteronilus Mensae.
The HiRISE database presented here provides a framework for more detailed stratigraphic correlations based primarily on physical characteristics observed in the rocks, but image-based stratigraphy has obvious limits, particularly concerning absolute age correlations. The future construction of an absolute geologic time scale for Mars will require geochronological studies carried out by rovers and landers at a local scale, i.e., Farley et al. [2014].
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