Clast arrangement influences fracture stratigraphy in a carbonate toe-of-slope megabreccia, Permian Rader Member, Bell Canyon Formation, Guadalupe Mountains, Texas, USA

Debris-dominated carbonate slopes comprise extensive volumes of margin-derived lithoclastic and skeletal components. While reefal margins and their steep slopes have been extensively studied and are actively targeted for hydrocarbon production, lower-angle distal slope settings remain less explored. Thus, there are limited insights into depositional processes, stratigraphic architecture, and fracture distribution within these settings, which all strongly influence fluid flow in the subsurface. To address these knowledge gaps, outcrop data were studied from the middle Rader Member megabreccia, a Permian carbonate toe-of-slope deposit exposed in Bell Canyon, Guadalupe Mountains, west Texas, USA. Multiscale sedimentological and structural observations, integrating drone-based digital outcrop models, photomosaics, scanlines, detailed graphic logs, and a novel square-meter grid-cell mapping method, were used to quantify internal architecture, clast-size distribution, matrix composition, and fracture patterns.

The studied outcrop, up to 25 m thick and exposed along ~700 m of depositional dip, exhibits repetitive lateral thinning caused by basal erosion and thickening due to the stacking of lithoclasts. Clast-size distributions show inverse grading of megaclasts (boulders exceeding 0.5 m) and normal grading of smaller clasts, along with repetitive downslope coarsening-to-fining patterns influenced by the presence of megaclasts. These observations suggest that the deposit at the studied locality was emplaced as a single event by a granular debris flow (i.e., a grain flow) with abundant particle collisions that promoted inverse grading of megaclasts.

Two fracture sets are present in the megabreccia, striking NE-SW and NW-SE, with broadly comparable dimensions and intensity. Fractures are rarely longer or taller than 0.5 m, with high aspect ratios, reflecting strong architectural confinement imposed by the inversely graded structure of the deposit. Larger and more dimensionally variable fractures occur preferentially in coarser, poorly sorted intervals, demonstrating textural control on fracture scaling. Preferential fracture propagation within the matrix of the megabreccia and frequent terminations at clast boundaries show that clasts act as mechanically competent inclusions, indicating that grain-to-matrix stiffness contrasts govern fracture growth. These findings show that extreme textural and mechanical heterogeneity fundamentally limits fracture growth, suggesting poor fracture-system connectivity in lower-slope carbonate megabreccias. The quantified relationships presented here aid the improvement of fracture-system prediction for reservoir models by incorporating the depositional processes and stratigraphic architecture that impact the resulting fracture geometries in carbonate megabreccia deposits.