Geology of the Monterey Bay Region

USGS open file report 77-718

by, H. Gary Greene


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Geophysical data and sea floor samples collected from the continental shelf and slope between Ano Nuevo Point and Point Sur  California indicate that the Monterey Bay region has had a complex late Cenozoic tectonic history. Uplift and depression have produced a succession of regressive and transgressive sedimentary units, while contemporaneous right-slip along faults of the San Andreas system have offset major structural and lithologic elements. This deformation produced three regional and several local unconformities within upper Tertiary rocks and initiated development of a canyon system that today includes the Monterey, Ascension, Carmel, and other large submarine canyons.

The Tertiary stratigraphy of the offshore Monterey Bay area is divided into two provinces by a major structural boundary, the north-trending Palo Colorado-San Gregorio fault zone. East of this zone in the offshore are four seismically distinct sequences that can be correlated with major sequences onshore. These sequences comprise (1) pre-Tertiary basement, and (2) middle Miocene, (3) upper Miocene to Pliocene, and (4) upper Pliocene to Holocene sedimentary intervals. Each of the latter three sequences is bounded by unconformities, as is its counterpart on land. Only Neogene sedimentary rocks are present offshore; Paleogene units, if originally present, have been removed completely by pre-middle Miocene erosion.

An extensive erosional surface was cut during Zemorrian time into the late Mesozoic granitic basement rocks. Incised into this surface are the ancestral Monterey Canyon and an unnamed canyon. Marine sedimentary rocks of upper Miocene and Pliocene age overlie this unconformity and fill the unnamed canyon. Similar rocks also may have once filled Monterey Canyon. Near shore these strata are covered by terrestrial alluvial and eolian deposits, deltaic deposits, marine canyon fill, landslide and slump deposits, and unconsolidated sediments that range in age from upper Pliocene to Holocene. Monterey Canyon appears to have been filled and exhumed at least twice since its inception in Oligocene time, once in late Miocene and once in Pleistocene time.

Three major seismic stratigraphic units are apparent in continuous seismic reflection profiles from the offshore area west of the Palo Colorado-San Gregorio fault zone. These are (1) acoustical basement, and (2) middle Tertiary and (3) late Tertiary to Quaternary sedimentary intervals. Acoustical basement comprises Cretaceous to early Tertiary sedimentary rocks, Mesozoic or older metamorphic rocks, and Cretaceous or Jurassic rocks of the Franciscan assemblage. The middle Tertiary sequence consists of sedimentary rocks of questionable Miocene age. The late Tertiary to Quaternary sequence is composed of Pliocene sedimentary rocks and unconsolidated marine sediments, and submarine landslide and slump deposits.

Seismic reflection surveys indicate two major, intersecting, northwest-trending fault zones to be present in the offshore Monterey Bay area. The Palo Colorado-San Gregorio fault zone may be more than 200 km long; it is narrow (approximately 3 km wide) and is represented in most places by one or two faults. This zone appears to connect with faults mapped on land near Ano Nuevo Point and Point Sur. The Monterey Bay fault zone, located in the area between Santa Cruz and Monterey, is a diffuse zone, approximately 10 to 15 km wide, of en echelon faults. Faults within this zone appear to connect with faults on land near Monterey, and the zone appears to be truncated by the Palo Colorado-San Gregorio fault zone west of Santa Cruz.

locations of more than 110 earthquakes (1968-1976) show that the newly mapped fault zones in Monterey Bay are seismically active. Epicenters in the bay form two clusters, one at the intersection of the Monterey Bay and Palo Colorado-San Gregorio fault zones, and the other in a linear belt that trends northwest along the Palo Colorado-San Gregorio fault zone. Faults within the Monterey Bay fault zone parallel the San Andreas fault, and fault-plane solutions of two earthquakes in this fault zone indicate right-lateral, strike-slip displacement, similar in sense to the San Andreas fault. Fault-plane solutions of six earthquakes associated with the Palo Colorado-San Gregorio fault zone also indicate right-slip. The Palo Colorado-San Gregorio fault zone parallels the Hayward fault and resembles that fault in length, seismicity, and sense of movement. Moreover, both faults appear to intersect the San Andreas fault, at the same angle, the Hayward fault at its south end and the Palo Colorado-San Gregorio fault at its north end.

Recent earthquake activity on the Palo Colorado-San Gregorio and Monterey Bay fault zones probably reflects stress release along the San Andreas fault system, of which these zones are considered a part. Displacement along faults within these fault zones and along the newly named Ascension fault also may explain the apparent discrepancy in total offset along the San Andreas fault system in central and southern California. Right-slip within these fault zones has slivered the Salinian block, producing a serrated western margin for the block. This serrated margin was fragmented as the Salinian block was displaced northwestward along the San Andreas fault; tectonic slivers were pushed ahead of the block or were carried along at its seaward margin. A model for the tectonic slivering and elongation of the Salinian block is proposed on the basis of the sense of movement and patterns of faulting observed in the central part of the block, in the Monterey Bay region. Right-slip along the Palo Colorado-San Gregorio fault zone and older (pre-Pliocene) Ascension fault probably has offset the lower part of Monterey Canyon almost continuously for the past 20 m.y. The displaced segments of the lower canyon were exhumed during Pleistocene time, and are represented today by Pioneer and Ascension canyons. The present distance between Pioneer and Monterey canyons, 110 km, is a measure of offset along these faults since middle Miocene time. Strike-slip faulting also may explain the presence of submarine canyons elsewhere that head well offshore, on the outer shelf and inner slope.