Using the modern Colorado delta to reconstruct the compound clinoforms of the Pliocene Colorado delta

Abstract

ABSTRACT Compound clinoforms contain both shoreline and subaqueous clinoforms, both with a topset–foreset–bottomset morphology, but with a wide subaqueous platform (30–150 km) in between. Recent datasets suggest that compound clinoforms are common in tide-dominated deltas, but there are only a few good examples from ancient deposits. The limited vertical expression of the shoreline clinoform and the lateral extent of the subaqueous clinoform hamper the identification of compound clinoforms in the rock record. The modern Colorado River Delta, in Baja California, Mexico, is tide-dominated and exhibits a compound clinoform; we use this delta as an analog to guide our interpretation of the ancient (Pliocene) Colorado delta in Southern California. The outcrops of the Deguynos Fm. are a representative section of the Pliocene Colorado delta. Four stratigraphic sections were measured from the outcrops, seven facies associations are recognized, and three depositional environments are interpreted using the nearby modern Colorado delta as an analog system. The Deguynos Fm. exhibits at least 22 parasequences organized in 4 stratigraphic parasequence sets; the parasequences (∼ 12 ky each) are likely autogenic stratigraphic responses, while the parasequence sets (∼ 62 ky each) are interpreted as a combination of eustatic changes and tectonic response to the creation of accommodation space along the West Salton Detachment Fault. The comparison between the Pliocene and modern Colorado deltas provides an excellent opportunity to better understand ancient compound clinoform deposits and to aid recognition of this morphology in other locations.