Wisconsinan Climate, Tectonism, and Origin of the Water Gap Channels on the Southern California Coastal Plain

Ivan Colburn
Dept. of Geological Sciences, California State University, Los Angeles, Los Angeles, CA

Six ephemeral rivers flowed across the Orange/Los Angeles coastal plain during the semi-arid Sangamonian Interglacial Age. These were the Los Angeles, Los Cerritos, San Gabriel, Bolsa Chica, Santa Ana, and Newport rivers. The semi-arid climate was succeeded by the Wisconsinan glacial/pluvial climate changing the rivers into large perennial ones with great erosive power. During the Sangamonian-Wisconsinan transition tectonic upwarping along the Newport/Inglewood fault zone elevated the Newport/Inglewood Ridge above the surrounding terrain. The growing ridge transected all six rivers, five of which successfully cut channels through the ridge as it rose beneath them. The successful rivers are known as antecedent ones and the channels through the ridge are known as water gaps. At Signal Hill the ridge rose too rapidly for the Los Cerritos River to complete a water gap across it. This river was diverted south to the San Gabriel River water gap, leaving a wind gap at Signal Hill. The Wisconsinan glacial climate lowered sea level 300 feet below present-day level. The increased rainfall during Wisconsinan time caused the remaining five rivers to erode their channels down to the new lower base level, thereby entrenching their courses in their own flood plains. Once entrenched, these rivers were constrained from shifting their courses laterally on their flood plains in order to cut additional water gaps in the ridge contrary to the assertions proposed by Stevenson and Emery(1958), Moran and Weibe (1992), and Lajoie and Ponti (1992). The low stand of the sea also forced these rivers to extend their courses two to 14 miles out onto the newly exposed Southern California marine shelf. The waning of the Wisconsinan pluvial/glacial climate raised sea level once more, causing the Holocene sea to advance inland in the entrenched river channels up to two miles from the present-day coast. Sediment carried by these rivers was deposited at the inland ends of the estuaries, forming deltas that prograded seaward gradually raising the floors of these channels up to present-day sea level.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California