LAUSR

Stein et al. (2018) described a striking synsedimentary deformation feature on bedding planes in ca. 3.6–3.2 Ga lacustrine mudstones on Mars. It consists of a small-scale network of polygonal, interconnecting, straight to arcuate sandstone ridges on a 1-cm-thick red mudstone overlying very fine-grained sandstone, visible in two adjacent outcrops called Old Soaker and Squid Cove. The polygons are 0.5–3.5 cm wide and mostly four-sided; the ridges are a few centimeters to ~0.3 m in length. Most of the ridge junctions are orthogonal; some ridges taper off. The ridges are the surface expression of cracks filled with material no larger than coarse silt, but compositionally similar to the underlying sandstone. Polygons also exhibit a subset of millimeter-wide fractures. Stein et al. interpret the networks as desiccation cracks and the filling coming from a presumed gray mud overlying the cracks. They suggested that lake levels oscillated such that there was intermittent subaerial exposure of the muds. This is not the first report of possible desiccation cracks on Mars (El-Maarry et al., 2014). Stein et al. rejected other crack-forming mechanisms, namely syneresis, which is passive; chemically induced shrinkage commonly considered to be salinity-related; and hydraulic fracturing caused by elevated fluid pressure, typically during burial. On the other hand, the shape of the polygons departs somewhat from the familiar, albeit diverse, patterns observed in modern muddy sediments on Earth. Moreover, the polygon margins show no evidence of curling or disintegration, which would be expected on a subaerial surface exposed to the elements, even under the early Martian atmosphere if it was moist or windy. Their sharp edges are clear from the ChemCam close-up images on Sol 15555 (https:// marsmobile.jpl.nasa.gov/msl/multimedia/raw/?s = 1555&camera = CHEMCAM%5F). The typical V-shaped cross section of desiccation cracks is also not evident, and instead, where the mudstone is broken away, the cracks appear to be parallel-sided (Stein et al.’s figure 4B, and the ChemCam images). Although not commented upon, the overlying bed in the Squid Cove outcrop also appears to possess a network of polygons, but they are larger (Stein et al.’s figure 4A). A fourth explanation for generating and filling cracks is that they are dewatering features that formed under shallow burial of wet sediment due to seismic shock. Earthquake-induced shaking causes cyclic pore pressure increase, which can lead to substratal liquefaction and injection of sediment into fissures that formed due to concomitant shrinkage of stratigraphically adjacent mud. Both upward and downward injection is observed. Earthly analogs for the Mars cracks—small dikes or dikelets— are legion, but those in mudstones of the ca. 1.45 Ga Belt Supergroup of western North America are particularly instructive and much admired (Fig. 1; Pratt, 1998, 2017). A wide variety of crack patterns is present on bedding planes, from reticulate to irregular polygons as well as linear and spindle-shaped forms. They often exhibit a hierarchy of two or three size groupings, and typically are different in successive beds. In vertical cross section, they are broadly parallel-sided. The material filling these cracks is upward-injected clay and silt from underlying layers. While they may superficially resemble desiccation cracks and are wildly heralded as such, they cannot be, and other evidence of subaerial exposure is lacking. Seismically induced soft-sediment deformation is expressed variably depending on lithology, because different types of sediment possess contrasting rheological attributes.