77-1 Boxworks in Terrestrial Sedimentary Environments

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

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Boxworks are common features in sedimentary rocks that result from a variety of formation mechanisms. These features are found in many different sedimentary lithologies such as limestones, siliciclastics, and salts, and range in size from micron- to kilometer-scale. The simplest are secondary sedimentary structures formed in unlithified sediment such as mudcracks (cm- to m-scale). These can then be infilled with sediment or cement that produce positive topographic features when overlying strata are eroded. Other formation mechanisms are initiated during burial diagenesis such as fractures with subsequent cementation or magmatic intrusions (mm-km scale). When exposed via erosion, these subsurface features become topographically upstanding boxworks. Recognizing the formation mechanism is crucial to understanding the depositional, diagenetic, and erosional history of the rocks, particularly because many of these mechanisms occur via water/rock interaction and thereby influence—and subsequently preserve the record of—subsurface habitability. This work explores the variety of means by which boxworks form in terrestrial environments to provide a framework for understanding similar features on other solar system bodies. The focus will be on the formation mechanism and identifying features such as morphology, mineralogy, and geochemistry. The mechanisms of emplacement, in turn, affect subsurface habitability. For example, when fractures form in the subsurface, they become preferential flow paths for reservoir fluids owing to enhanced permeability relative to the surrounding host rock. When reservoir fluids meet and mix with fluid(s) of different chemistry, cement minerals can precipitate. These fractures become hubs of habitability compared to the host rock because of increased and focused fluid and energy source influx (e.g., redox reactions for chemotrophs). Alternatively, if the boxworks are caused by intruding magma, the surrounding metamorphic halo is sterilized. Nevertheless, any time an ecological niche is created by an extinction, organisms rush in to colonize and evolve in that space. Combined with the incursion of elements from the magmatic fluid, the zone surrounding magmatic intrusions would become a hyper-habitable environment as soon as it cooled to temperatures conducive to life.

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