GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 132-2
Presentation Time: 1:45 PM


MUHLBAUER, Jason G. and FEDO, Christopher M., Department of Earth & Planetary Sciences, University of Tennessee, 1621 Cumberland Avenue, 602 Strong Hall, Knoxville, TN 37996-1526

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.