IS THE PRESENCE OF QUASI-CIRCULAR DEPRESSIONS (QCDS) INDICATIVE OF SEDIMENTARY MATERIALS?

Quasi-circular depressions without a structural representation in Viking and MOC visible imagery (invisible QCDs) have been identified in MOLA topography data. It has been suggested that these features represent completely buried impact craters. Topographic depressions will form over impact craters buried by a cover material that is differentially compacting because total cover thickness, and thus total percent compaction, is greater over the center of completely buried impact craters than over their rims. The linear relationship of invisible QCD surface relief to diameter supports differential compaction models of formation. If differential compaction is the process by which invisible QCDs form, then only areas of differentially compacting materials should have invisible QCDs. As the definition of differential compaction requires sedimentary accumulation, we can then also assume that invisible QCDs only form in sedimentary regions.

Previous observations show that invisible QCDs around the Utopia, Isidis and Acidalia basins occur exclusively in the hummocky member (unit AHvh) of the Vastitas Borealis Formation (unit VBF). Unit AHvh is interpreted by Tanaka et al. [2003] as outflow-channel sediments whose pervasive alteration has been aided by subsurface volatiles; this interpretation is consistent with a differently compacting material. No invisible QCDs are identified in the volcanic units of Utopia, Isidis or Acidalia, also as predicted by differential compaction models. Volcaniclastic units in these regions, which are expected to differentially compact and could thus form invisible QCDs, are all heavily disrupted by violent volatile release, which likely erase the shallow depressions.

We explore the geographic distribution of invisible QCDs 7-100 km in diameter throughout the northern lowlands. We identify the geologic units as mapped by Tanaka et al. [2003] and determine which could differentially compact. We compare QCD location to geologic units and materials and observe that invisible QCDs are located only in those surface units where differential compaction is possible.