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Sedimentation of the Sespe Formation in Southern California
The Sespe Formation includes both fluvial and deltaic elements. Two sedimentation cycles characterize the Sespe Formation basin-wide and are represented by fining-upward megasequences, each hundreds of meters in thickness. The megasequences very likely reflect two episodes of global eustatic sea level change. Each megasequence consists of a coarse basal conglomerate with a general upward-thinning of strata and an upsection decrease in particle size.
The lower Sespe megasequence was deposited by very competent, high-capacity, west-flowing, braided fluvial and flash flood systems in an alluvial coastal plain that prograded seaward from late-middle to late Eocene time. These units grade westward into delta distributary channel and lacustrine delta fill deposits. The major sedimentary regression recorded by the lower Sespe megasequence coincides temporally with a globally recognized eustatic sea level drop at the end of Eocene time.
The upper and lower Sespe megasequences are separated by a major erosional unconformity that is well defined in some areas. Evidence for this major unconformity includes: i) a mappable erosional unconformity between braided fluvial deposits containing late Oligocene vertebrates no older than about 30 million years which overlie deltaic deposits containing late Eocene marine mega- and micro fossils; and ii) the absence of vertebrate fossils of Chadronian and Orellan (Early Oligocene) age in the Sespe Formation.
The apparent absence of early Oligocene strata and the initiation of a major sedimentary transgression during late Oligocene and earliest Miocene (upper Sespe Vaqueros) time coincides with the eustatic sea level drop which occurred about 31 million years ago. Subaerial erosion during this low-strand of sea level may have removed the early Oligocene part of the section from some or all of the basin.
The upper Sespe megasequence in the main part of the Sespe basin was initially deposited by a west-flowing, braided fluvial system with short-distance, proximal, braided streams or alluvial fans along the northern margin of the basin. An upsection evolution to meandering stream deposition takes place basin-wide.
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