Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Tectonics And Sedimentation Of The Mio-Pliocene Ranger Zone Of The Puente And Repetto Formations, Los Angeles Basin, CA


The integrated effect of differential subsidence and variable sedimentation rate in a transform margin setting is well documented in two Mio-Pliocene, stratigraphic sequences of the Ranger Zone (Pico and Repetto Formations) that subcrop at Wilmington oil field on the west margin of Los Angeles basin, CA. Ranger deposition, concurrent with growth of Wilmington anticline, was on the seaward side of a 40-km wide, line-sourced, shore-parallel, turbidite complex that was confined in between the paleoshoreline and a thrust-cored paleohigh — the Palos Verdes Hills. Although the main turbidite complex prograded from NW to SE parallel to shore, the Ranger prograded SW across Wilmington anticline and seaward, with concurrent lateral migration — perhaps due to opening of a wider outlet to the sea through the Palos Verdes paleohigh. Sand-rich Ranger units on the crest of Wilmington anticline correlate to thicker shale sections on the flanks, an architecture resulting from differential subsidence during deposition. Interestingly, structurally-controlled thickening and thinning of strata is most evident in sand-poor sections, and least evident in sand-rich sections, even though Wilmington anticline and associated normal faults likely existed throughout deposition. Instead, sandy sections thicken along presumed NE-SW sediment transport directions, and cross the axis of the anticline, whereas shale sections thin along NW-SE trends parallel to the axis. Thus, structural controls (e.g., subsidence) are most evident in sand-poor sections, and stratigraphic controls most evident in sand-rich sections. Most likely, the presence and/or absence of a structural versus stratigraphic signature reflects sedimentation rate. Because sand-rich sections originated as debris flows and high-density turbidity currents, sediment accumulation was rapid. By contrast, sand-poor sections dominated by suspension settling, and dilute remnants of turbid flows, took longer to deposit similar sediment thicknesses. Thus, enough time existed during shale deposition for anticlinal growth and fault displacements to affect sediment accumulation. By contrast, only evidence of sediment transport directions was preserved when sand flows overwhelmed the system, despite ongoing tectonic subsidence. Thus, the sequence architecture reflects the integrated effects of both controls, and sedimentation rates must be considered if using stratal thicknesses to infer the timing of structural events.