Sedimentology and diagenesis of the boxwork-hosting Altadena Member, Gale Crater, Mars

Since May 2025, the Mars Science Laboratory Curiosity rover has been investigating a distinctive stratigraphic interval in the sedimentary “Mount Sharp” in the center of Gale crater (Fig. 1a). This unit was defined from orbit as the “boxwork unit” due to a rectilinear network of erosion-resistant ridges ~5 meters across with central fractures intersecting to form polygonal hollows ~10–20 meters wide and 1–2 meters deep (Fig. 1b). Boxwork was mapped across 14 km with ~100m vertical relief. On the ground, Curiosity’s traverse upsection through this region has included imaging and in situ chemical data on ridge tops, margins (“ridge walls”), and topographic lows (“hollows”), as well as drilled samples from a ridge and adjacent hollow for in situ mineralogy. The unit is defined as the Altadena member of the Mirador formation.

The original orbital interpretation for the boxwork pattern formation involves (1) deposition and lithification of the “host” strata, (2) fracturing, potentially contemporaneous with (3) fluid flow under pressure allowing percolation into rock on either side of fractures, creating (4) a cementation or alteration zone around the fracture, followed by (5) eolian erosion of weaker hollows. On the ground, the host rock is dark, friable, and fine-grained with faint mm-scale planar laminations, provisionally interpreted as lacustrine. Surprisingly, chemical differences are not observed between ridges and hollows, but there are significant differences in diagenetic features: ridges are dominated by blocky laminated mudstone with abundant white veins (Fig. 1f), with round ~5mm nodules increasing in abundance towards ridge walls and down into hollows (Fig. 1d, e), which are often associated with heavy overprinting creating nodular, flakey, or dendritic morphologies. Mineralogical differences were noticeable; ridges are phyllosilicate-rich, whereas hollows contain siderite. We will learn more about the depositional and diagenetic history of the Altadena member as Curiosity continues to traverse this unique unit.