Fine-Scale Sedimentary Architecture of the Jezero Western Delta Front
Résumé
A key exploration target for the Perseverance rover mission is a sedimentary fan deposit at the western margin of Jezero crater, which has been interpretated to be a river delta that built into a lake basin during the Late Noachian-Early Hesperian epochs on Mars (~3.6-3.8 Ga). Images from the Mastcam-Z and SuperCam Remote Micro-Imager instruments provide striking views of the stratigraphy exposed in the fan’s erosional front. Here, we report its stratigraphy and sedimentology, which place constraints on the nature of the fan deposits and their paleoenvironmental implications.
Multiple Mastcam-Z mosaics show spectacular views of the stratigraphy of a prominent embayment in the delta scarp that has been informally named Hawksbill Gap, the lower section of which the rover is investigating and sampling. The prominent cliffs of the eastern and western margin of Hawksbill Gap show distinctive stratal geometries with complex stratigraphic relations. The basal succession comprises poorly exposed thinly bedded, planar laminated sandstones that are interpreted as the deposits of low-density turbidity currents. A locally prominent, resistant unit named Rocky Top comprises planar stratified pebbly sandstones also likely to be high-density turbidite deposits. The stratigraphic mid-sections of the scarps are characterized by packages of decameter-scale inclined tabular strata. These tabular beds are locally conglomeratic but predominantly comprise finer-than-conglomerate lithologies, likely pebbly sandstones. Interstratified within these are poorly sorted matrix-supported conglomerates interpreted to be debris flow deposits. The inclined strata are overlain across a sharp truncation surface by generally planar parallel thin-bedded horizontal strata that we interpret as topset beds. Conglomerate beds containing bouldersare located within the overlying topset strata. The stratal patterns are broadly consistent with deposition in a Gilbert-type delta setting with basal strata representing deposition from sediment gravity flows, inclined strata representing foreset beds, and overlying topset beds deposition from fluvial processes in a delta top environment. The boulder conglomerates indicate sediment-transport on the delta top by episodic high-discharge floods.