LONG-LIVED DEEPWATER ANTIDUNES: OUTCROP DESCRIPTION OF LOW-ANGLE UNDULATING, SYMMETRIC, UPFLOW ACCRETING BEDFORMS WITHIN SUPERCRITICAL DOMINATED SLOPE DEPOSITS IN THE FISH CREEK-VALLECITO BASIN, LATE MIOCENE GULF OF CALIFORNIA

Antidunes are a fixture in the Froude supercritical realm of bedform stability diagrams but are generally short-lived features in modern open channel settings. Interpreted antidune outcrop examples are thus restricted to environments conducive to rapid deceleration and sedimentation such as washover fans, glacial outburst flows, and pyroclastic ash-surges. In deepwater environments, evidence from modeling, seafloor bathymetric studies, and direct turbidity current observation increasingly suggests that sediment gravity flows often reach Froude supercritical flow states along moderately steep basin-margin slopes. While turbidity currents are net depositional, antidunes have generally been assumed not to preserve in deepwater deposits out of bias to their open channel ephemerality.

Spatially extensive slope turbidites of the Fish Creek-Vallecito Basin in the Anza Borrego Desert State Park of southern California reveal a variety of bedsets containing low-angle, symmetrical, undulating bed geometries that stack opposite to paleocurrent indicators. The turbidites, developed on steep marginal slopes during the initial opening of the Gulf of California (Late Miocene), contain three primary facies: 1) 10-45 cm thick, medium-grained, normally graded sandstone beds, 2) 1-15 cm thick, fine-grained sands interbedded with silty mudstones, and 3) 50-100 cm thick laminated silty mudstones. 5-10 m-thick bedsets are composed of ~20-30 sigmoidal beds. Beds emerge tangentially as thin, flat, and fine-grained sediments that then transition downflow into thicker, coarser, upstream inclined sediments before transitioning back into thinner, finer deposits that flatten and pinch out or recline downflow as a full waveform. Waveforms are 3-7 m in amplitude and 75-100 m in wavelength with dip angles generally <10°. Bioturbated, 1-3 cm-thick mudstone caps to sandstones indicate that bedding geometries and stacking patterns were sustained through time across flow events. Deposits are interpreted as antidunes, the first such-recognized preserved, long-lived, large-scale antidunes in a deepwater setting.