Macropaleontology and biostratigraphy across the Cretaceous/Tertiary boundary, San Francisquito Formation, Warm Springs Mountain, Los Angeles County, Southern California

A biostratigraphic and lithostratigraphic analysis of the lowermost 165 m of the upper Maastrichtian to middle Paleocene San Francisquito Formation at Warm Springs Mountain, north of Castaic Lake, Los Angeles County, southern California, shows a conformable section across the Cretaceous/Tertiary {K/T) boundary. The lowermost 165 m of the formation represents a transgressive sequence that grades vertically upward from rocky intertidal, to shoreface, to transition zone, to offshore deposits. An underlying granite-gneiss basement and an overlying submarine-channel conglomerate stratigraphically bound the sequence. This sequence is between 68.5 Ma and 63.8 Ma in age, based on the Turritella subspecies zonation of Saul {1983a). Deposition of this transgressive sequence probably resulted from a combination of a subsiding basin and an eustatic transgression. The K/T boundary is within an 8 m-thick interval composed of shoreface, transition zone, and offshore deposits. This interval lies between the transition zone and the offshore environments. Position of the K/T boundary is defined by the Campanian-Maastrichtian Roudairia peruviana? Olsson, the late Maastrichtian Turritella chaneyi orienda Saul, and the Danian Turritella peninsularis quaylei Saul. A brief shallowing event recorded at the K/T boundary at Warm Springs Mountain may have resulted from an eustatic regression that occurred at that time. If there is any hiatus at the K/T boundary at this locality, it represents a very small time interval. Thirty-one taxa have been found at Warm Springs Mountain. These taxa are represented by 11 gastropods, 17 bivalves, two ammonites, and one shark. Two probable new species of bivalves have been found. Species composition of the shallow marine macrofauna changes across the K/T boundary. This observed change may be due to either environmental changes resulting from deepening water or to the Cretaceous/Tertiary mass extinction, or to both. 2