ABSTRACT

The paleomagnetism and heavy mineral assemblages of sandstones from the middle and upper members of the Sespe Formation were studied to clarify stratigraphic relations. Magnetostratigraphic data, and an upsection provenance change reflected in heavy mineral suites, indicate a major intraformational unconformity within the upper member of the Sespe Formation. The hiatus represented by this unconformity apparently spans much or all of the late Eocene and early Oligocene (approximately 29.5-36 Ma). In Simi Valley, the late Uintan and Duchesnean (Chrons C19n-C16r, 41.2-36.5 Ma) and the early Arikareean (Chrons C10r-C9r, 28.0-29.5 Ma) are the only intervals represented by strata. These relations indicate that the Sespe Formation was deposited much more rapidly and discontinuously than previously thought. The most prominent unconformity is attributed to a middle Oligocene eustatic sea level drop. The paleomagnetic results support previous interpretations that the western Transverse Ranges province was technically rotated clockwise by ∼90° during the late Cenozoic.

INTRODUCTION

The Sespe Formation is a sequence of terrestrial clastic rocks typically sandwiched between middle Eocene and lower Miocene marine rocks. It consists of braided fluvial, deltaic, and alluvial fan deposits that crop out in several fault-bounded blocks in southwestern California (Fig. 1). The Sespe Formation is also found in the subsurface offshore and crops out locally on Santa Rosa Island south of the Santa Barbara Channel. Thus, its geographic distribution defines a large non-marine basin elongated in an east-west direction.