Sesimic sequence stratigraphy of late Miocene to Pliocene sediments in the Elk Hills area, southern San Joaquin Basin

by Sophia Cobos, Oxy

ABSTRACT

In the southern San Joaquin basin, the late Neogene was a time when sediment supply kept pace with relative sea level change to allow the accumulation of a thick section of shallow-marine sediments, which make up the Etchegoin and San Joaquin Formations.

For late Miocene to Pliocene sediments of the Elk Hills area in the southern San Joaquin basin, the interpretation of seismic reflection patterns allows the identification of eight surfaces, defined in this study as sequence boundaries, which bound seven stratigraphic sequences from the base of the Etchegoin Formation to the base of the San Joaquin Formation. Electric-log stacking patterns and core descriptions within these sediments confirmed the cyclicity observed through seismic reflection profiles.

Thickness variations in time isochron maps of the stratigraphic sequences indicate that the growth of the Elk Hills subsurface anticlines was a major depositional control during the time of deposition of the late Miocene to Pliocene sediments of the Elk Hills area.

The predominant coarsening-upward stacking pattern within the individual stratigraphic sequences, shown by electric logs and core samples, indicate that relative sea level rise was a major control during the deposition of the Etchegoin and San Joaquin formations in the area of study. Faulting also controlled thickness variations in later stages.

The overall upward increase in grain size, from the base of the Etchegoin Formation to the base of the San Joaquin Formation, indicate that sediment input was incrementally higher than relative sea level rise, allowing the basin to be gradually filled, and the environmental conditions to transition from marine-shallow marine, during the deposition of the Etchegoin Formation, to transitional brackish-shallow marine, during the deposition of the San Joaquin Formation. Sand distribution maps indicate that much of this increase in sediment was derived from the Kern River which flowed into the study area from the Sierra Nevada range to the east.