Supercritical‐flow structures in a Cretaceous submarine channel–lobe transition zone, Point Loma Formation, California

Abstract Submarine fan deposits are important archives of Earth's history and serve as the record of turbidity current events that transfer large amounts of terrestrial sediment and carbon into the ocean. The geomorphology, depositional processes and facies architecture change significantly along the submarine‐fan depositional profile, particularly at the channel–lobe transition zone (CLTZ). However, few studies have documented detailed facies architecture and associated sedimentary structures in the CLTZ, which are important for paleoenvironmental interpretations and morphometric information essential to reconstructing sediment transport dynamics and reservoir connectivity in ancient successions. Excellent coastal‐cliff outcrops from ancient CLTZ deposits of the Upper Cretaceous Point Loma Formation in San Diego, California constrain the facies architecture of the CLTZ and the three‐dimensional geometry of supercritical‐flow bedforms. We update previous work on sand‐rich and mud‐rich lobe complexes at Sunset Cliffs and present the first description of the uppermost sand‐rich CLTZ complex that contains antidune deposits arranged in predominantly metre‐thick tabular packages. Paleocurrent data suggest a compensatory evolution between elements and complexes that was probably driven by updip avulsions. The data generated by this study help to characterise the detailed facies architecture and bedform geometry for CLTZ deposits, which enables a better understanding of the sediment transport dynamics of the channel–lobe transition zone. These data, when combined with other ancient and modern analogues, can be used to predict reservoir connectivity and heterogeneity in CLTZ deposits that form hydrocarbon reservoirs and potential carbon storage sites.