Geology and surrounding recent marine sediments of Anacapa Island

Title:: Geology and surrounding recent marine sediments of Anacapa Island
Abstract:: "Anacapa Island lies nearly twelve miles off the southern California coast along latitude 34 00'30"N. It is the easternmost member of the four Santa Barbara Islands and is also the closest to shore of the eight California channel islands. The island is approximately five miles in length but is only 1.05 square miles in area. Two gaps divide the island into smaller eastern, central, and western islands. The western island has a summit elevation of 930 feet, and it is the highest and largest of the three. Drainage is principally to the north by intermittent resequent streams. Prominent wave-cut terraces are present at elevations of about 600 and 250 feet. The higher terrace is present only on the western island, but the lower terrace occurs on each of the islands and forms a summit platform atop the central and eastern islands. The insular Anacapa shelf includes about thirty-six square miles of the surrounding shallow submerged sea floor. North of the island the shelf is approximately three miles wide, but the southern shelf is less than half this width. To the west, the shelf area extends beneath Acapaca Passage and merges with the shelf of neighboring Santa Cruz Island. The shelf area east of the island is poorly defined. Declivities of the northern and the southern shelves average about 1 degree 40' out to a depth of about 270 feet. The outer edge of the southern shelf usually occurs at this depth, but north of the island a two-mile wide terrace with an overall gradient of 0 degrees 5' extends to near the shelf edge, which occurs at an average depth of 300 feet. Other flattenings of the shelf are present at depths of 60, 100, 180, and 330 feet. The submarine terraces are thought to be wave-cut features of Wisconsin age. Lower Middle Miocene andesitic lavas and pyroclastic rocks of the Conejo formation form the bulk of Anacapa Island. Approximately 1700 feet of this formation is exposed on the south side of the western island. Near the base of the volcanic series are two interbeds of San Onofre breccia, which are separated from each other by 100 to 150 feet of eruptive rocks. This formation crops out on the western and central islands; each unit of the formation has a maximum thickness of thirty five or forty feet. Fossiliferous Lower Pleistocene terrace deposits overlie the San Onofre breccia on the central island. The marine deposits are overlain in turn by Upper Pleistocene and Recent nonmarine terrace sands. The geology of the submerged insular shelf is similar to the island proper except for outcrops of the Monterey formation beneath Anacapa Passage. Most submarine outcrops occur on the shelf areas east and west of the island, where a thin veneer of Recent and relict Pleistocene sediments is present. A paucity of rock bottom north of the island indicates a thick cover of Recent sediment. Anacapa is essentially a gentle north-dipping (2-20 degree) sequence of volcanic rocks, and it is structurally related to the other Santa Barbara Islands and to the Santa Monica Mountains. The islands and the Santa Monica Mountains form the structural province of Anacapia and also the western portion of the Transverse Range Physiographic Province. Unconsolidated sediments overlying the insular shelf are characterized by an average median diameter of 0.360 mm, a sorting coefficient of 1.73, a carbonate content of 45.5 percent, and a 0.7 percent organic matter content. Calcium carbonate and organic matter increase with distance from the island. Median diameters decrease with increasing depth north and south of the island, but increase with distance to the east and west. Detrital sediments or deposits which have carbonate contents less than fifty percent occur principally north and south of the island. They are coarsest to the south because grains smaller than about 0.125 mm tend to bypass the narrow and rather steeply sloping southern shelf. Calcareous sediment types are characteristic of the eastern and western shelf areas where an environment of nondetrital sedimentation prevails. Most of the calcareous matter is composed of mollusk fragments, Lithothamnium debris, bryozoan zoaria, and echinoid spines and plates. Foraminifera also contribute, but they are most abundant in the sediments near the outer edges of the northern and southern shelves.
Supplemental_Information:: Samples collected aboard the R/V Velero IV; Data digitized by the USGS for inclusion into usSEABED (<http://walrus.wr.usgs.gov/usseabed>)

Range of values
Minimum:	-2.0
Maximum:	4.8
Units:	Phi
Resolution:	0.1

Range of values

Minimum:

-2.0

Maximum:

4.8

Units:

Phi

Resolution:

0.1

Range of values
Minimum:	5.1
Maximum:	93.9
Units:	Percent
Resolution:	0.1

Range of values

Minimum:

5.1

Maximum:

93.9

Units:

Percent

Resolution:

0.1

Range of values
Minimum:	0.27
Maximum:	3.98
Units:	Percent
Resolution:	0.01

Range of values

Minimum:

0.27

Maximum:

3.98

Units:

Percent

Resolution:

0.01

Geology and surrounding recent marine sediments of Anacapa Island

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