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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 438

Last Page: 439

Title: Channel-Levee-Overbank Sequence in Paleocene Submarine Canyon Fill, Point Lobos, California: ABSTRACT

Author(s): H. Edward Clifton

Article Type: Meeting abstract


Vertical sequences characterized by upward decreases in the grain size and bed thickness of turbidites are commonly attributed to laterally migrating channel-levee-overbank systems. A probable Paleocene example of such a sequence is superbly exposed at Pebbly Beach, Point Lobos, California. Contact relations indicate that the Paleocene deposits fill a steep-walled sinuous valley carved into underlying granodiorite of late Cretaceous age. The few fossils found in the Paleocene rocks indicate deposition in water depths of 100 m (328 ft) or more and suggest that the sediment accumulated in a submarine canyon. Although most of these Paleocene deposits are conglomeratic, the upper part of the section exposed at Pebbly Beach consists of a 30-m (98-ft) thick fining-upward sequen e from conglomerate through sandstone to mudstone. About 10 m (33 ft) of predominantly thick-bedded sandstone grades upward through a transitional sequence of about 2 m (6 ft) of thin-bedded sandstone into

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more than 15 m (49 ft) of mudstone with thin sandstone interbeds. The thick-bedded sandstone beds in the lower part of the sequence are typically several tens of centimeters thick, graded, and display Bouma Ta-b or Ta-b-c intervals. Although many of these sandstone beds are visibly lenticular, the associated thin mudstone interbeds extend laterally across the exposure, unchanged in thickness or lithology. This thin-bedded transitional sequence consists of graded sandstone beds, 5 to 15 cm (2 to 6 in.) thick, that displays Bouma Tb-c intervals. The mudstone unit that caps the succession consists of bioturbated mudstone with thin (mostly less than 5 cm, 2 in.) sandstone interbeds. Paleocurrents in the upper and lower parts of the succession (predominantl as recorded in ripples and ripple bedding) trend dominantly southwest. The transitional sequence, however, displays a consistent 30° deflection of currents to the south; in addition, these strata are inclined slightly more steeply to the south than the beds above or below.

I interpret this overall fining-upward sequence as resulting from lateral migration of a channel within the submarine canyon. The thick-bedded sandstone beds represent deposition on the flank of the channel; the lenticularity of these beds suggests that they were not deposited by a pure turbidity current but were carried by additional (or other) mechanisms, such as fluidized flow. The mudstone interbeds reflect predominantly pelagic deposition in the intervals between sediment gravity flows. The thin-bedded sandstone sequence represents a levee facies deposited primarily from tractive currents associated with the gravity flows that spilled over the channel and were deflected slightly to the south by the slope on the outer side of the levee. The upper, predominantly mudstone part of th section was deposited as pelagic sediment interspersed with overbank flows that traveled down the general slope of the submarine-canyon floor. An erosional surface with 3 to 4 m (10 to 13 ft) of relief near the top of the thick-bedded sandstone, covered by a mudstone breccia talus, records an episode wherein the channel reversed its migration direction and locally eroded the upper channel-flank deposits.

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