A stratigraphic framework of the Jezero upper fan succession observed in an erosional window at Gnaraloo Bay, Jezero crater, Mars.

The NASA Perseverance rover has been traversing the Jezero western fan, a clastic succession on the western rim of Jezero crater, containing a series of rocks deposited between ~3.6-3.8 Ga that show evidence of basinward prograding fluvial-deltaic depositional conditions.

The lowest stratigraphy in the Jezero fan records a transition from igneous crater floor material to distal deltaic deposits. A transition to fluvio-deltaic and periodic debris flow deposition is recorded in the upper fan series: The Tenby formation sandstones are comparable to terrestrial meandering fluvial systems, planar-bedded coarse sandstones of the Otis Peak member overlie the Tenby formation, and a blocky unit of boulder deposits, referred to as the Boulder Unit tops the fan. To the west and north, the upper fan overlies a carbonate-bearing sandstone deposited on the crater rim: the Margin Unit. The contacts between these units have been obscured for much of the traverse, precluding detailed assessment of their stratigraphic relationships.  

Gnaraloo Bay, visited on Sols 959 – 1000 of the mission, is an erosional window where the upper fan intersects the Margin Unit. Erosion through three key stratigraphic elements presents an opportunity to unravel the relative timing relationships of the Jezero crater rim and upper fan. We present a stratigraphic framework built from observations in Gnaraloo Bay made from images collected with the Mastcam-Z stereo-camera system.

The majority of Gnaraloo Bay is formed of shallow dipping (<10°) packages of beds dipping either towards or away from the crater rim, part of the Margin Unit. Erosional truncations are present where packages are juxtaposed and one interpretation amongst others is that these are comparable to shoreline deposits in smaller terrestrial lacustrine settings.

An abrupt erosional boundary at Airey Hill separates outcrop of the Margin Unit and Tenby formation recording an abrupt transition to an initial phase of channelized upper stage flow directly on top of the Margin Unit, followed by deposition of increasingly thick migrating barforms.

The lower flanks of Vancouver Point expose sub-horizontal, well bedded, rough textured sandstones comparable to the Otis Peak member. The basal contact of these sandstones crosscuts and postdates both the Margin Unit and Tenby formation. The upper <5 m of Vancouver Point is topped by the Boulder Unit with a basal contact that downcuts the Otis Peak member beds, implying a time gap between member deposition.

The basal contact of the ~ 600 m linear ridge of the Boulder Unit at the Jurabi Point ridge crosscuts both the Margin Unit and Tenby Formation, indicating an erosional unconformity. The Otis Peak member is absent here, implying that it is not associated with Boulder Unit deposition, unconformably overlying the Margin Unit and Tenby formation, and pre-dating the Boulder Unit.

We interpret the stratigraphy at Gnaraloo Bay to record the initial deposition of channelized migrating barforms over rim-bounding margin deposits. This was followed by periodical fan progradation and subsequent deposition of sheets of the Otis Peak member which appear to have been shielded from erosion by late Boulder Unit debris flow deposition sourced from Neretva Vallis.