Polygonal fractures in the Siccar Point group: evidence for late, near-surface fluid cycling in Gale crater, Mars

Abstract:

Martian habitability has historically been discussed in the context of ancient terrains that contain evidence for surface or near-surface interaction with liquid water. In Gale crater, potentially habitable environments have been described within the fluvio-lacustrine strata that comprise the Bradbury and Murray formations. Unconformably overlying these formations is the Siccar Point group (Stimson formation), which has been interpreted to record deposition in a considerably younger, dry aeolian environment. The potential for fluid stability, however, is expressed within Siccar Point strata through a laterally extensive network of erosionally resistant polygonal fractures. Analysis of polygon morphology across ~20 km² of the Siccar Point group revealed that polygons (N=8714) are uniform in size and express unique modes of intersection at 90 and 120 degrees (N=642). Additionally, polygons are stratigraphically restricted to a relatively thin, lower horizon within the Siccar Point group. Together, these data are consistent with fracture formation under contractional processes within a regionally homogenous substrate. Given sufficient cohesion of the ~meter-thick aeolian deposit, we propose that these polygonal fractures represent the interaction of lower Siccar Point group strata with near-surface groundwaters and the range in intersection angles supports repetitive wet-dry contractional cycles. The erosional resistance of polygons indicates cementation by later diagenetic fluid flow. The formation and subsequent cementation of polygonal fractures supports the intermittency of wet environments within relatively young strata in Gale crater.