STRATIGRAPHY OF SÉÍTAH: UNDERSTANDING THE OLDEST GEOLOGIC UNIT EXPOSED IN THE JEZERO CRATER FLOOR

The Perseverance rover has landed in Jezero crater, Mars. Séítah is an exposure of the oldest geologic unit on the Jezero crater floor. This unit contains olivine and hydrated minerals that record volcanism, sedimentation, diagenesis, and aqueous alteration on ancient Mars. Evaluating the depositional origin/timing and diagenesis of this unit will improve our understanding of the geologic evolution and habitability of rocks in the Perseverance field site. Jezero crater was previously a lacustrine environment, and its surrounding region contains a widespread aqueously altered olivine-bearing rock unit that may be an ashfall deposit. Here we use images from SuperCam and Mastcam-Z onboard the Perseverance rover to identify facies in Séítah and constrain its geologic history.

Rover observations of Séítah show several distinct facies cropping out within NE-SW-trending, decameter-high ridges. Identified facies include thin (≤ ~1-3 cm) beds with a nodular appearance, erosionally resistant beds of variable (~10-40 cm) thickness, massive rocks, and rocks with large nodules. No definitive clasts or grains have been observed in long-distance images with resolutions of ~2-3 mm/pixel, suggesting subcentimeter grain sizes. An origin as aeolian dune deposits is not favored given the lack of cross-bedding, and fluvial channel geometries have not yet been observed. Planar bedding of variable thickness within the Séítah ridges could be consistent with clastic deposition from suspension coupled with deposition from higher-energy flows in a lacustrine setting. This interpretation would also be consistent with the occurrence of these outcrops within an ancient crater lake basin, basinward of the Jezero delta. However, the potential orbital map correlation of the Séítah outcrop with the regionally widespread olivine-bearing unit outside of Jezero raises the possibility of a subaerial volcaniclastic or airfall origin. Further distinction between subaqueous and subaerial depositional origins for Séítah likely requires fine-scale observations of grain-size, fabric, and textures by the rover’s science instruments upon closer approach. Rocks formed from subaerial and subaqueous deposition have different biosignature preservation potential, making the characterization of depositional environment important.