SEDIMENTOLOGY OF A CAMBRIAN BRAID DELTA DEPOSIT, MIDDLE MEMBER WOOD CANYON FORMATION, MARBLE MOUNTAINS, CALIFORNIA

In the central part of Marble Mountains of southeastern California, the Cambrian (Terreneuvian) middle member Wood Canyon Formation preserves evidence of interplay between braided-fluvial, tidal, and wave deposition in a distal, flat, braid-delta plain. Lithologic and stratigraphic data contrast with middle member rocks exposed six km away, where braidplain sandstones dominate absent marine influence. An ~80 m thick, bed-by-bed stratigraphic column of the middle member in the central part of the Marble Mountains divides into four depositional units. Meter-scale stratigraphic panels traced on high-resolution Structure-from-Motion models, with bounding surface orientations, reveal the architecture of braid-delta elements.

Fluvial features of the middle member include trough cross-bed cosets of meter-scale thickness that represent mostly vertically accreting barforms (VA). Inclined cosets that represent downstream accretion elements (DA) are pervasive. Rare, massive, meter-thick sand beds may represent channel fill (CH) elements. The influence of tidal activity is found in thick (1-20 cm), homogeneous mudstones, interbedded with pebble conglomerate, that resemble those left by fluid-mud “soupgrounds.” Fluid-muds, commonly reported in river deltas and tidal channels alongside thalweg gravels, form near the turbidity maximum at the fresh and salt water interface. Wave-related elements include reworked light-colored sand waves and well-preserved symmetrical and interference ripples.

Lowest in the stratigraphy (0-20 m), Unit 1 is fluvial-dominated, with a consistent paleoflow direction, DA elements, gravel lags, and minimal mudstone beds. Unit 2 (20-35 m) displays distinct marine influence, including more variable paleocurrent, thick mudstones with rare mud cracks, reduced conglomerate occurrence, ripple beds, meter-scale tangential sets of cross-bedding, and rare trace fossils. The majority of the middle member is Unit 3 (35-60 m), which displays an increase in pebble conglomerate, chaotic mud-clast horizons, and ripple surfaces. Increased maximum grain sizes, rip-up beds, and abundance of large DA and CH elements may suggest a return to more fluvial-dominated conditions. Unit 4 caps the middle member (60-80 m) with facies that are broadly similar to Unit 2.