Fluvial to deltaic clinoforms observed by Curiosity in Gale crater’s Mount Sharp

The Curiosity rover continues its exploration of Mount Sharp, Gale crater’s ~5000 m-high sedimentary pile, and has been traversing for the past three years the Layered Sulfate unit (LSu), an interval initially characterized from orbit and thought to have recorded a global climatic transition toward the more arid conditions we observe nowadays on Mars. This unit, also informally known as the Mirador formation, is rich in sulfates and record mostly aeolian settings. Unexpectedly, the rover has also encountered numerous strata arguing for a recurring aqueous activity punctuating the overall arid, aeolian depositional environment.

Lately, Curiosity explored the “boxwork” unit, a high-interest region named after the orbital observation of “box-forming”, (deca-)meter-scale rectilinear features cropping out of the ground. Diagenetically-altered, fine-grained rocks making the most of the boxwork unit are probably of lacustrine origin, stressing out the importance of these aqueous conditions in the midst of the LSu. But when looking at the walls of this valley, made up of the Texoli, Mishe Mokwa and Cordillera buttes, we notably observe coarser-grained, erosion-resistant beds displaying a wealth of multi-scale sedimentary structures.

Among them are several occurrences of clinoform geometries that we sorted into three classes. Type 1 are characterized by inclined, sigmoidal to poorly cross-bedded strata, filling meter-scale, individualized lens-shaped bodies. Type 2 are characterized by inclined strata, sigmoidal but more cross-bedded strata. They are also observed filling lens-shaped bodies, but contrary to Type 1, these lenses are laterally stacked and cross-cutting each other’s immediate neighbor. Finally, Type 3 clinoforms occur in unconfined packages evidencing clearly sigmoidal, steeply-dipping (15-20°) and non-cross-bedded strata. While they are conformable with lower sub-horizontal layers pertaining to the bedrock, their top is mostly truncated by unconformable sub-horizontal layers. At the outcrop, the steeply dipping, sigmoidal strata also define a conspicuous lobate shape.

We interpret Types 1 and 2 clinoforms as the record of fluvial channels, with Type 1 a record of braided rivers and Type 2 a record of laterally migrating bars of a meandering river. Type 3 marks a conspicuous change and we interpret the vertical tripartite stratal pattern as bottomsets, foresets and topsets of a Gilbert-deltaic suite. These strata reflect fluvial to deltaic depositional settings with decreasing levels of energy from strictly fluvial, individual channels (Type 1), meandering channels (Type 2) and finally within a delta (Type 3).

These settings are in line with the quieter, presumably lacustrine, environment the boxwork unit’s strata likely origin from, and could represent the local sedimentary input. They contrast with the overall arid, aeolian structures observed to make most of the surrounding buttes and overall LSu. They nevertheless highlight a recurrence of humid episodes throughout the LSu. These events illustrate a more complex and unpredictable climatic pattern as Mars became colder and more arid.