Extensive Polygonal Fracture Network in Siccar Point group Strata: Fracture Mechanisms and Implications for Fluid Circulation in Gale Crater, Mars

Abstract

Rock fractures are indicators of stress release within geologic systems, and fracture morphologies can commonly be used to infer formation conditions. Polygonal fractures are common in isotropic, contractional stress regimes such as in rocks exposed at the surface of a planet undergoing thermal cycling or in sedimentary substrates undergoing repeated wetting and drying. Such polygonal fracture systems, on centimeter to decameter scales, have been widely documented on Mars. Utilizing a combination of orbital-and ground-based images, we report a laterally extensive polygonal fracture network that occurs within siliciclastic rocks of the lowermost Siccar Point group, Gale crater, Mars. The Siccar Point group is exposed over approximately 20 km 2 in northwest Gale crater, where it unconformably overlies eroded strata of Mount Sharp (Aeolis Mons) and reflects likely aeolian deposition along the lower flanks of Mount Sharp. Images reveal an extensive network of erosionally resistant polygons, approximately 7.5 m across, that exhibit interior angles (i.e., fracture intersections) with modes at 90°and 120°. Polygon morphology indicates that fractures formed during multiple cycles of expansion and contraction, which is attributed to desiccation and subsequent recharge of near-surface groundwater. The erosional resistance of preserved fractures is inferred to reflect postfracture diagenetic fluid flow along the sub-Siccar Point group unconformity and cementation. Evidence for multiple fluid events in the relatively young strata of the Siccar Point group requires a protracted history of fluid stability in Gale crater.Plain Language Summary Deciphering the processes that affect sedimentary rocks after their deposition is critical to understanding the geologic history of a basin. Fractures occur when applied forces exceed the strength of the host material, and we can infer the process by which fracturing occurred by assessing the morphology of fractures. In this study, we analyze a network of polygonal fractures within rocks of the Siccar Point group, a relatively young geologic unit that is exposed over~20 km 2 of Gale crater, the field site of the Curiosity rover. Polygons formed by these fractures are similarly sized and intersect at angles that show dominant modes at 90°and 120°. These observations suggest that fractures formed under conditions of uniform stress and likely result from contractional processes such as climate-driven wet-dry cycles. The cementation of fractures by later fluids then imparted erosional resistance that permits these features to be recognized from orbit. The presence of an extensive, fluid-driven fracture system within aeolian strata highlights the potential complexity of martian climate signals.

Synergistic Ground and Orbital Observations of Iron Oxides on Mt. Sharp and Vera Rubin Ridge

“…A third observing mode, along-track oversampled (ATO), is achieved by adjusting the angular velocity of CRISM's gimbal to collect 18 m/pixels at intervals <18 m in the along-track direction. ATO data can be processed to an along-track resolution of 9-12 m/pixel using an iterative maximum-log likelihood (MLM) algorithm, which retrieves the best estimate of scene radiance in the presence of noise (L. Kreisch et al, 2017). The MLM processing technique can also be applied in the spectral dimension for HRL, FRT, and ATO images to de-noise spectra .…”

“…We used MLM-processed CRISM ATO (used to enhance both the spectral and spatial dimension), FRT, and HRL (spectral only) scenes that had also been atmospherically corrected using the Discrete Ordinates Radiative Transfer Program for a Multi-Layered Plane-Parallel Medium (DISORT) (L. Kreisch et al, 2017). Using DISORT, we modeled the joint radiance contributions from the Martian surface and atmosphere, including the scene-dependent effects of aerosols.…”

Extraformational sediment recycling on Mars

“…After most of the sedimentary rock that used to lie between the crater walls and Aeolis Mons was broken down and removed from Gale crater (Supplement S1), forming the moat that separates the mountain from the crater walls, a record of subsequent sedimentation was captured in the rocks of the Siccar Point group. These rocks include sediments deposited in Gediz Vallis (Malin and Edgett, 2000;Palucis et al, 2016;Fraeman et al, 2016) and dark-toned, erosion-resistant rocks that are today exposed in patches that unconformably drape across the older Aeolis Mons strata (Fraeman et al, 2016;Kronyak et al, 2019a). Rocks of the Stimson formation, part of the Siccar Point group, contact the Murray formation and have been explored at several locations using the Curiosity rover ( Fig.…”

“…2D;Watkins et al, 2016). Recent observations suggest that the Stimson formation could truncate >500 m of Murray formation strata , and, certainly, Siccar Point group rocks, as a whole, drape across more than 500 m of Mount Sharp group strata (Fraeman et al, 2016;Kronyak et al, 2019a). The paleorelief across this buried surface generally follows the slope of the modern, exposed Murray formation surface (i.e., sloping northward) but locally varies at scales of 5-25 m over the region in which Stimson formation outcrops occur (Watkins et al, 2016).…”

“…Inside Gale crater, after the Stimson formation eolian sands were in place, conditions on Hesperian Mars were such that the sands were preserved in the sedimentary record. To be retained, they were wetted (Kronyak et al, 2019a), buried, and lithified.…”