About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 512

Last Page: 513

Title: Neogene Fore-Arc Basin Development in Northern California: Eel River Basin: ABSTRACT

Author(s): Julie S. Menack, Steven B. Bachman

Article Type: Meeting abstract


Strata representing the youngest phase of fore-arc basin development in northern California are exposed in an unusual cross-sectional view across the basin axis. The exposure of a large part of the stratigraphic section of the Neogene Eel River (Humboldt) basin can be attributed to uplift along the northern edge of the Mendocino triple junction. The fore-arc deposits overlie the Mesozoic and Cenozoic accretionary prism and slope deposits of the Franciscan Complex. Outcrop geology of the uplifted southern flank of the basin indicates that the facies and sediment distribution patterns agree with paleobathymetric studies; a complete deep to shallow marine transition is recorded in the basin sediments.

Facies studies demonstrate the time-transgressive nature of the sedimentary environments. Proximal facies are landward (east) of coeval deeper water deposits exposed along the coast. The basal contact is clearly depositional on this southern flank of the basin. Sandstones and pebbly conglomerates cut into coastal belt Franciscan accretionary prism sediments inland. A previously undescribed debris flow is conformable on similar Franciscan sediments along the coast. This debris flow is faulted against overlying faulted and fractured basin plain siltstones and fine sandstones (Miocene-Pliocene) which contain thin lenses of shell debris, pebbles, and glauconitic sand. A monotonous accumulation of organic-rich diatomaceous mudstones is capped by amalgamated channels continuing sequences of thin glauconitic sands with locally derived siltstone rip-ups, siltstone, and hemipelagic mudstones. The overlying sediments consist of fine-grained turbidites, thick bioturbated siltstones and fine sandstones, and coarser turbidites. A continental shelf sequence concludes this phase of Eel River basin development.

To the south of the main basin outcrop, progressive uplift and faulting related to the migration of the triple junction have left erosional remnants of sediments coeval with Eel River basin rocks. These rocks are found up to 50 km (31 mi) south of the upturned basin edge, suggesting that the basin was at one time more extensive to the south. Shallower depositional environments in some of these basin remnants may indicate the proximity of the original southern edge of the basin. Structural complexities to the south include strike-slip faulting and possible upper Miocene and younger trusting of Franciscan melange over Neogene marine sediments.

End_Page 512------------------------------

Shallow gas fields have been developed in the basin; ongoing exploration for deeper oil and gas is not yet definitive. Potentially good source rocks in the deeper parts of the basin, underlying organic-rich Franciscan sediments, and the abundance of potential reservoir rocks higher in the section make this structurally complex onshore/offshore basin an attractive exploration target.

End_of_Article - Last_Page 513------------

Copyright 1997 American Association of Petroleum Geologists