GEOLOGICAL MAP OF THE EQUATORIAL LAYERED DEPOSITS OF THE FIRSOFF CRATER AREA: STRATIGRAPHIC CONSTRAINTS AND FACIES VARIABILITY

The Equatorial Layered Deposits (ELDs) of of the Firsoff crater area (including some smaller craters and the surrounding plateau) have been mapped in order to understand the stratigraphic relations with the other deposits and to characterize their facies variability and distribution.

The ELDs stay nonconformably on top of the Plateau Sequence of Noachian age (Scott & Tanaka, 1986) both within and outside craters. Outside the craters they are unconformably covered by the Ridged Plains Material of Hesperian age (Scott & Tanaka, 1986).

ELDs form 100s meters thick-bulges within craters and drape the plateau showing a flat-lying geometry outside the craters (Pondrelli et al., 2011). Within the craters, they consist of two units: a meter-scale layered unit disrupted in a post-depositional polygonal pattern and 100s meters large mounds made of breccia (Pondrelli et al., 2011). Layers seem to continue from the mounds to the layered unit suggesting a facies heteropy between the two units. Mounds often possess an orifice at the top consistent with an origin related to fluid expulsion (Pondrelli et al., 2011). Both layered unit and mounds are often associated to possible fissure ridges suggesting sourcing from the subsurface. Polyhydrated sulfates have been detected using CRISM data on the layered unit in the crater east of Firsoff.

We suggest that the Firsoff crater ELDs might have formed as spring deposits because of 1) the large scale geometry (bulge with maximum thickness roughly on top of the central peak of the crater); 2) the presence of morphologies suggesting fluid escape; 3) sulfate composition and sedimentary structures are consistent with evaporite precipitation.

Outside of craters, ELDs are still characterized by high albedo and layering but are found associated to duneforms and show meter-scale cross stratification. The scale of the cross stratification and the overall morphological setting suggest an aeolian depositional environment for at least part of the ELDs located outside of craters possibly associated to playa deposition as in Meridiani Planum (Grotzinger et al., 2005).

References: GROTZINGER, J. et al. (2005). EPSL, 240, 11-72. PONDRELLI, M. et al. (2011). EPSL, 304, 511-519. SCOTT, D. & TANAKA, K. (1986). US Geol. Surv. Misc. Invest. Ser., Map I-1802-A.