ABSTRACT Application of updated diatom biochronology to the Monterey Formation and related biosiliceous rocks reveals the imprint of both global paleoclimatic/paleoceanographic and regional tectonic events. A rise in global sea level combined with regional tectonic deepening associated with the development of the transform California margin resulted in the abrupt onset of deposition of fine-grained Monterey sediments that were relatively free from clastic debris between 18 and 16 Ma. The base of the Monterey Formation does not mark a silica shift in diatom deposition from the North Atlantic to the North Pacific Ocean. Rather, a North Atlantic Ocean decline of diatoms after ca. 13 Ma and increasing divergence in nutrient levels between the North Atlantic and North Pacific Oceans between ca. 13 and 11 Ma coincided with a major enhancement of diatom deposition in the Monterey Formation. A stratigraphically condensed interval of phosphate-rich sediments between 13 and 10 Ma in coastal southern California appears to have resulted from sediment starvation in offshore basins during a period of higher sea level, as inland sections such as those in the San Joaquin Valley commonly contain thick sequences of diatomaceous sediment. Increasing latitudinal thermal gradients in the latest Miocene, which triggered a biogenic bloom in the equatorial Pacific Ocean at 8 Ma, also led to enhanced diatom deposition in the uppermost Monterey Formation and overlying biosiliceous rocks. Uplift of the California coastal ranges after ca. 5.2 Ma resulted in an increasing detrital contribution that obscured the presence of diatoms in onshore sediments. Major reduction in coastal upwelling in the early Pliocene ca. 4.6 Ma then caused a drastic reduction of diatoms in sediments offshore southern California.

ABSTRACT Forearc basins are first-order products of convergent-margin tectonics, and their sedimentary deposits offer unique perspectives on coeval evolution of adjacent arcs and subduction complexes. New detrital zircon U-Pb geochronologic data from 23 sandstones and 11 individual conglomerate clasts sampled from forearc basin strata of the Nacimiento block, an enigmatic stretch of the Cordilleran forearc exposed along the central California coast, place constraints on models for forearc deformation during evolution of the archetypical Cordilleran Mesozoic margin. Deposition and provenance of the Nacimiento forearc developed in three stages: (1) Late Jurassic–Valanginian deposition of lower Nacimiento forearc strata with zircon derived from the Jurassic–Early Cretaceous arc mixed with zircon recycled from Neoproterozoic–Paleozoic and Mesozoic sedimentary sources typical of the continental interior; (2) erosion or depositional hiatus from ca. 135 to 110 Ma; and (3) Albian–Santonian deposition of upper Nacimiento forearc strata with zircon derived primarily from the Late Cretaceous arc, accompanied by Middle Jurassic zircon during the late Albian–Cenomanian. These data are most consistent with sedimentary source terranes and a paleogeographic origin for the Nacimiento block south of the southern San Joaquin Basin in southern California or northernmost Mexico. This interpreted paleogeographic and depositional history of the Nacimiento block has several implications for the tectonic evolution of the southern California Mesozoic margin. First, the Nacimiento forearc depositional history places new timing constraints on the Early Cretaceous unconformity found in forearc basin strata from the San Joaquin Valley to Baja California. This timing constraint suggests a model in which forearc basin accommodation space was controlled by accretionary growth of the adjacent subduction complex, and where tectonic events in the forearc and the arc were linked through sediment supply rather than through orogenic-scale wedge dynamics. Second, a paleogeographic origin for the Nacimiento forearc south of the southern San Joaquin Valley places new constraints on end-member models for the kinematic evolution of the Sur-Nacimiento fault. Although this new paleogeographic reconstruction cannot distinguish between sinistral strike-slip and thrust models, it requires revision of existing sinistral-slip models for the Sur-Nacimiento fault, and it highlights unresolved problems with the thrust model.