77-3 Formation of the Boxwork Unit in Gale Crater, Mars

Session: Boxwork and Fracture Halos: Changes in mineralogy and erosion resistance around fracture features on Earth, Mars, and across the Solar System

Presenting Author:

Kirsten Siebach

Authors:

Siebach, Kirsten Leigh¹, Seeger, Christina², Mondro, Claire A.³, Hausrath, Elisabeth⁴, O'Connell-Cooper, Catherine⁵, Dietrich, William Eric⁶, Schwenzer, Susanne⁷, Gasda, Patrick⁸, Thompson, Lucy Margaret⁹, Scuderi, Louis¹⁰

(1) Rice University, Houston, TX, , (2) Rice University, Houston, TX, USA, (3) Planetary Science Institute, Knoxville, TN, USA, (4) University of Nevada, Las Vegas, NV, USA, (5) University of New Brunswick, Fredericton, NB, Canada, (6) University of California, Berkeley, CA, USA, (7) The Open University, Milton Keynes, UK, United Kingdom, (8) Los Alamos National Lab, Los Alamos, NM, USA, (9) University of New Brunswick, Fredericton, NB, Canada, (10) University of New Mexico, Albuquerque, NM, USA,

Abstract:

The Mars Science Laboratory rover Curiosity explored the orbitally-described “boxwork unit” on Mt Sharp in Gale crater between May 2025 and March 2026 (mission Sols 4534-4844). This stratigraphic interval, ~14 km across and ~10s-100 meters thick, is distinguished by the presence of rectilinear intersecting erosion-resistant ridges ~5 meters across that create a network of polygons ~10-20 meters across with ~1-2 meter-deep hollows. Based on orbital observations, the ridges were interpreted as fractures and fracture-adjacent rocks that were strengthened relative to hollows by groundwater cementation, with relatively recent erosion highlighting the contrasting rock strengths. We will discuss how Curiosity’s observations inform our current interpretation of the formation of the boxwork unit, following a temporal sequence of steps:

(1)   Deposition and Lithification: On the ground, the rocks that host the orbitally-mapped boxwork are dark-toned, friable, fine-grained, and planar laminated, making them sufficiently distinct from the surrounding light-toned, sandy, cross-stratified Chenapau member rocks to define a new Altadena member of the Mirador formation. The Altadena member is interpreted to have formed in lakes that were likely contemporaneous with surrounding sand sheets (Seeger et al, this meeting). Based on mineralogy in a hollow floor, the lakebeds include detrital igneous silicates cemented by siderite, with some amorphous materials.

(2)   Fracturing: After deposition, lithification, and some degree of burial, rectilinear fractures formed within the Altadena member lake beds, with fractures occasionally crossing into adjacent sandy rocks (Mondro et al, this meeting).

(3)   Ridge-Strengthening: After fracturing, while the Altadena member was still buried, groundwater fluids migrated through the fracture system. The fluids had the strongest influence on rock within ~2 meters of the fracture conduits, creating more massive textures with occasional large nodules, and diminishing influence further away from the fractures, with small nodules or flaky textures. The fluids did not dramatically change the chemical composition of the bedrock as measured by APXS and ChemCam, but did dissolve some of the detrital silicates and siderite cement to generate phyllosilicates and iron oxides.

(4)   Erosion: The “boxwork” pattern is visible today due to eolian erosion that preferentially erodes more friable or poorly lithified materials. Turbulent eddies exploit heterogeneities to make incipient hollows that grow deeper and wider into 1-2 meter deep hollows. North-south winds create elongated hollows along Curiosity’s traverse.

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