HYDRAULIC FRACTURING OF MARTIAN MUDSTONES

The Mars Rover Curiosity has landed on an ancient sedimentary succession. Between December 2012 (Sol 126) and June 2013 (Sol 298) it explored a mudstone bearing interval in Yellowknife Bay (YB), approximately 450 m east of Bradbury Landing. Though the primary interest was the information that these rocks contain about past habitability and organic matter preservation, they also contain late diagenetic calcium-sulfate filled fractures that relate to their burial history. Sharp contacts between vein fills and host rock indicate that the rock was consolidated at the time of vein formation, and sub-horizontal and horizontal fractures suggest that the fluids from which Ca-sulfates precipitated exceeded lithostatic pressure.

On Earth, hydraulically fractured shales show crosscutting mineralized fractures (quartz, carbonate, sulfate minerals etc.) that vary in width from sub-mm to decimeter, and deformation due to subsequent compaction. The latter can inform about depth of burial when fracturing occurred, and suggests that hydraulic fractures can form in low permeability sediments at burial depths as shallow as 500-1000 m. For vertical fractures to form, the fluid pressure has to exceed the local minimum principal stress and the tensile strength of the rock, and when horizontal and/or bedding plane parallel off-shoots of fractures occur the fluid pressure has to exceed the local lithostatic pressure.

Hydraulically fractured mudstones on Earth and presumed Martian counterparts compare well in fracture style, deformation (ptygmatic folding), and associated concretions. With Mars gravity 62% lower than on Earth, a mudstone on Mars would require substantially deeper burial to achieve the same level of consolidation as an Earth equivalent. Thus, the soft and tightly compacted mudstones of YB may have been buried to substantial depths (in excess of 1 km) by the time they were hydraulically fractured. To generate the substantial volumes of mineralizing fluids that must have passed through these rocks probably requires a thick section of compacting sediments beneath YB. In summary, hydraulic fractures in YB mudstones attest to substantial burial and vertical movement of evolved brines from underlying or laterally adjacent strata.