Abstract

Located at the southern end of the Salton Sea in Imperial County, California, the New River Delta has a subaqueous extent greater than 25 km². The accumulating deltaic sediments are supplied by the New River at a rate of 500 × 10⁶ kg suspended sediment annually. New River drains 6,500 km² over its 150 km length.

Because of its unusually small size, the New River Delta provides an excellent model for a detailed analysis of deltaic facies distribution and relationships. Subaerial deposits comprising distributary channel, levee, interdistributary subaerial flat and crevasse deposits are the most varied. Subaqueous deposits are largely prodelta clay and delta-front fine silt.

Sediment size distribution is related to distributary patterns. Sand and coarse silt characterize distributary channel and proximal delta front environments. Lateral gradations from sand to clay or sand to interbedded silt and clay are common as one moves from distributary channels to interdistributary areas. Vertically, basal prodelta clays grade upward into silty delta front facies which either are overlain by interdistributary and marsh clays and silts, or by sands of distributary channels.

Sediment mineralogy reflects the composition of its source material—Colorado Delta detritus. Organic carbon is characteristically high (5%, maximum) in fine marsh clays, mudflat, and prodelta clays and low in delta front and distributary coarse silts and sand (0.58% minimum).

Climatic factors exert considerable control on deltaic configuration and the subaerial extent of the New River Delta has been severely reduced by rising water levels. The delta has accumulated sediment at an average rate of slightly over 1 cm/yr for the past 66 years. When the total volume of sediment in the delta, 0.024 km³, is compared with river discharge, it is apparent that most of the sediment is trapped by the delta.

Factors common to most modern deltaic masses such as abundant plant remains, high mica content, elongate sand bodies, highly organic prodeltaic deposits, and laminated sands over muds can be used in recognition of ancient deltas. All of these features are manifest in the facies of the New River Delta and indicate that similar balances between sediment input and local energy levels will produce similar sedimentary patterns regardless of scale.