Abstract: Late Neogene Paleobathymetry and Paleoenvironments of Humboldt Basin, Northern California

James C. Ingle, Jr.

Late Neogene marine sediments exposed in sea cliffs south of Centerville Beach, Humboldt County, California, record the filling of the southern part of the Humboldt (or Eel River) basin. Planktonic foraminifers, calcareous nannoplankton, and radiolarians indicate that this 1,900-m-thick sequence was deposited during early Pliocene (zone N18) through early Pleistocene (zone N22) time encompassing a span of 3 to 4 m.y. Quantitative analyses of well-preserved benthic foraminifers indicate that water depth shoaled from 2,000 to less than 100 m during this period. Radiolarian-rich diatomaceous mudstones of the Pullen Formation contain a lower bathyal benthic foraminiferal biofacies and represent early Pliocene basin-plain deposits. Thick turbidite sands of the Eel River Format on and the lower Rio Dell Formation overlie the Pullen Formation and contain lower bathyal through middle bathyal foraminiferal biofacies together with significant percentages of displaced upper bathyal, shelf, and littoral species. The abrupt appearance of this coarse terrigenous debris marks the rapid progradation of submarine-fan deposits onto the southern basin plain during late Pliocene time and the initiation of the final phases of basin filling. The middle member and lower part of the upper member of the Rio Dell Formation comprise a well-preserved Pliocene-Pleistocene basin-slope environment free of coarse turbidite sands and characterized by middle through upper bathyal foraminiferal biofacies, siltstones, and slump structures. Lower slope deposits contain only minor percentages of displaced benthic species, whereas upper slope and shelf deposits contain increasing percentages of neritic foraminifers and display increasing sediment size and traction structures, all recording the westward migration of the Pliocene-Pleistocene shelf prior to middle and late Pleistocene tectonic flexing of the basin margin. This same series of sedimentary events apparently occurred in deeper and more extensive parts of the Humboldt basin now deformed and underlying the submerged continental margin northwest of Cape Mendocino.

AAPG Search and Discovery Article #90976©1976 AAPG-SEPM-SEG Pacific Section Meeting, San Francisco, California