Geometry, architecture, and facies of an ancient sandy delta-scale compound clinoform in the Cozy Dell Shale of southern California, U.S.A.

ABSTRACT Ancient compound-delta clinoforms remain poorly documented in the stratigraphic record despite their growing recognition in modern continental-shelf systems. In combination with sandstone petrography and detrital-zircon geochronology, we describe the facies architecture, geometry, and stacking patterns of a compound delta clinoform, composed of a subaqueous-delta clinoform and an overlying subaerial delta front, in the middle Eocene Cozy Dell Shale of southern California. The intra-shelf subaqueous clinoform sets consist of multiple stacked lobes with sigmoidal geometries and facies transitions from distal hyperpycnite-dominated deposits to storm-influenced foresets, capped by bioturbated silts representing the subaqueous platform. The overlying sandstones contain stacked coarsening-upward parasequences interpreted as subaerial delta-front deposits. Paleocurrent measurements indicate that sediment was sourced from a southeastern river source and redistributed basinward and along strike by wave-supported gravity flows and storm-generated currents. These findings provide one of the few detailed outcrop-based facies models for ancient sand-prone subaqueous clinoforms, highlighting the role of storm-wave resuspension and gravity flows in constructing subaqueous foresets. The stratigraphic architecture of the delta-scale compound clinoform in the Cozy Dell Shale demonstrates the potential for sandy subaqueous clinoforms to accumulate at depths approaching and below storm wave base, underscoring the risk of misinterpreting such deposits as shallow-water shorelines in sea-level reconstructions.