Abstract

Cretaceous-Paleocene deposits of the Gualala basin and overlying deposits of the Neogene Point Arena basin record the complex tectonic and depositional history of the California continental margin as it changed from a convergent tectonic regime to a translational regime during the evolution of the San Andreas transform fault system. The Gualala and Point Arena deposits are among the northernmost onshore exposures of upper Cretaceous and Cenozoic marine strata west of the San Andreas fault and, as such, constitute a critically important but not intensely studied succession of rocks. In this paper, lithologic, stratigraphic, and paleobathymetric data from the Gualala and Point Arena stratigraphic successions are used to produce quantitative geohistory analyses that illustrate the subsidence and uplift of a portion of the California continental margin over the past 80 million yr.

The history of subsidence and uplift revealed through geohistory analysis of the Gualala and Point Arena basin-fill deposits clearly reflects two episodes of basin development directly associated with the convergent and transform phases in the evolution of the California margin. Beginning in Late Cretaceous time (∼80 Ma), the Gualala basin underwent slow rates of tectonic subsidence (∼30 m/m.y.). In late Paleocene time (∼57 Ma), tectonic subsidence increased to ∼140 m/m.y. and was accompanied by rapid rates of sediment accumulation (∼440 m/m.y.) that continued through the Eocene until terminated by basin filling, cessation of subduction tectonics, and flexing and uplift of the continental margin. Initial development of the Neogene Point Arena basin occurred in latest Oligocene time (∼24 Ma), some 2 to 4 m.y. after the birth of the San Andreas transform system. The earliest history of the Point Arena basin was marked by volcanism and extremely high rates of tectonic subsidence (∼1,060 m/m.y.) characteristic of a transtensional origin.

Based on our analysis, the tectonic and depositional histories of the Gualala and Point Arena basins can be summarized as follows: (1) during Late Cretaceous to middle Eocene, submarine fans of the Gualala and German Rancho Formations were deposited in a >2,000-m-deep subsiding basin that probably formed as a fore-arc basin in a setting characterized by oblique convergence and subduction along the California margin; (2) flexing and erosion of Cretaceous-Paleocene bathyal deposits of the Gualala basin occurred in the Oligocene epoch as the Pacific-Farallon spreading ridge approached the California margin, and as the accretionary prism outboard of the Gualala fore-arc basin migrated landward; (3) initial development of the Point Arena basin in earliest Miocene time (∼24 Ma) was accompanied by extrusion of the Iversen Basalt during the transitional period between convergent and transform tectonic regimes; (4) rapid subsidence of the Neogene Point Arena basin during Miocene time was related to transtension associated with the San Andreas fault, a pattern common to many of the small pull-apart basins formed along the California margin at this time; (5) early deposition in the Point Arena basin was dominated by submarine-fan deposits of the lower Miocene Skooner Gulch and Gallaway Formations, whereas siliceous biogenic sediments of the overlying Point Arena Formation signalled reduced deposition of coarse-grained terrigenous sediment during middle to late Miocene time; and (6) margin flexure and uplift of the Point Arena basin occurred in middle Pliocene (∼3.5 Ma) and early Pleistocene (∼1.6 Ma) time as a function of transpressional deformation of the crust in response to Pacific-North American plate interaction.

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