Anatomy of deltaic compound clinoforms : Colorado Delta in Baja California, Mexico, Pliocene Colorado Delta in Fish Creek-Vallecito Basin, California, USA, and a review of modern examples

Abstract

Modern Delta systems commonly exhibit compound clinoforms that consist of two elements, a shoreline clinoform, and a separate subaqueous clinoform, both with a topset–foreset–bottomset morphology, with a wide low-gradient area called subaqueous platform (30-150 km wide) in between. Recent datasets on modern tidal Deltas suggest that a compound-clinoform model is characteristic for tide-dominated Deltas but there are few good examples from ancient deposits. The modern Colorado River Delta, in Baja California, Mexico, is tide-dominated and exhibits a compound clinoform. The 20 km-long outcrops of the Yuha and Camel Head members of the Deguynos Formation, located in Fish Creek-Vallecito Basin, California, are the ancient (mid-Pliocene) deposits of the paleo-Colorado Delta. These outcrops were measured, producing 4 stratigraphic sections through the Yuha and Camel Head members. From these measured sections 7 facies associations were recognized. These facies associations were assembled into 3 depositional environments: subaqueous foreset to bottomset, subaqueous topset, and platform, and shoreline clinoform, using as an analogy the modern Colorado Delta deposit. The Yuha and Camel Head Members exhibit multiple at least 22 high-frequency regressive-transgressive (R-T) cycles in which thickness changes laterally along the outcrops. R-T sequences, with an upward-coarsening lower part and a sandy to silty upward-fining upper part are interpreted as 5th order sequence related to autogenic changes within the environment, as suggested by variations in thickness and approximate time duration of each cycle. The 5th order R-T sequences are further organized in four sequences interpreted as 4th order sequence, mapped by vertical changes in thickness and environment along the outcrop, and probably controlled by eustatic processes. The exceptional setting of the basin (strongly dominated by tidal processes and the uncommonly high accommodation rate) probably was the key to the preservation of the compound clinoform morphology in the rock record of the Fish Creek-Vallecito basin. To better understand sediment dispersal in tidal influenced Deltas, 138 bathymetry maps near River mouth were examined, finding 54 compound clinoform morphologies. The annual sediment flux (Mt/y) appears to be the most important factor in the preservation of the compound clinoform morphology. All River mouths with an annual measured sediment flux higher than 100 Mt/yr have a compound clinoform. In contrast, in River mouths with less than 15 Mt/yr, the compound clinoforms are rare. The 39 systems were divided into tide-dominated, wave-dominated, and mixed, following the averages measured values of tidal ranges and wave height from public databases. In these systems, the subaqueous platform area has a direct relationship with the annual sediment influx