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Using seismic records and porosity-depth curves, we calculate the volume of "dry" post-rifting sediment in the 4-m.y.-old central and southern Gulf of California (Fig. 1) to be approximately 12,500 km. For an area of 127,000 km, this volume represents a mean sedimentation rate of 2.5 cm/ky. for dry, solid material. Postrifting sediments on the proto-Gulf shelves (65,000 km) may fill a 15,000 km volume (a mean sedimentation rate of 5.8 cm/ky. for dry, solid material). Previous investigations of surface sediments and the results from holes drilled on DSDP Legs 64 and 65 allow us to estimate the contribution of principal sediment types, components, and grain size to the sediment budget for the southern and central Gulf. It has been assumed that vertical fades changes are insignificant. The results of our calculations are listed separately for postrifting sediments on oceanic crust and those on proto-Gulf shelves (Tables 3 and 4). For the study area as a whole, the following results may indicate the correct order of magnitude for the sediments deposited during the last 4 m.y.: 1) Sediment type (volume % of dry, solid sediment): shelf, 22%; slope, 37%; basin, 41%. 2) Texture (wt.%): sand, 13%; silt, 48%; clay, 39%. 3) Components (wt.%): "clay," 56%; quartz (terrigenous), 8%; biogenic silica, 14%; calcium carbonate, 7%; organic carbon, 2.3%; others, 13%. Note that the contribution of the three principal sediment types to the total sediment volume does not correspond with the area (as a percentage) of the shelves, slope, and basins (28:51:21) at present (Fig. 6). Redepositional processes transporting sediments, chiefly from the slopes to the basins, explain this discrepancy. Average sedimentation rates for the total study area are, for dry, solid material: 1) Total sediment, 3.6 cm/ky. 2) Terrigenous material, 2.5 3) Biogenic silica (opal), 0.5 4) Biogenic carbonate, 0.25 5) Organic matter, 0.35 If we consider the growth of the 4-m.y.-old postrifting Gulf from zero to its present extent, its mean sedimentation rate (5 cm/ky.) is approximately the same as that on the proto-Gulf shelf (5.8 cm/ky.). From the input of terrigenous material, the mean denudation rate of the drainage area (271,500 km) of the southern and central Gulf has been approximately 2 cm/ky. for the last 4 m.y. This rate is lower than most denudation rates that have been determined from sediment-load measurements for river basins with similar climates and relief. The characteristics of the drainage basin under study and causes for this discrepancy are discussed.

Budget of Post-Rifting Sediments in the Gulf of California and Calculation of the Denudation Rate in Neighboring Land Areas