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Abstracts: Speckled Beds in Diatomites of the Miocene Monterey Formation: Distinctive Gravity Flow Deposits in Finely-Laminated Diatomaceous Sediments


"Speckled beds" (SB) are abundant in finely-laminated diatomites of the Monterey Formation, at Celite diatomite quarry, Santa Barbara County, California. SB are non-uniformly distributed in a 60-m measured section, rarely exceed 10 cm in thickness, and have sharp basal and upper contacts. SB are characterized by discrete diatom-rich and detritus-rich blebs uniformly distributed in a mud matrix. In vertical section, the blebs are lenticular in shape, range from <1 mm to 1 cm in length, and are oriented parallel to bedding. In horizontal section, large diatom-rich blebs have a circular or irregularly rounded form. Proportions of the two bleb types is variable. Hand sample descriptions, thin section analyses, and scanning electron microscopy were used to interpret the origin of speckled beds.

The occurrence and subsequent disruption of strongly cohesive, finely laminated diatomaceous sediments is essential to SB formation. Laminated diatomaceous sediments situated on a subaqueous slope were disturbed by slumping, gliding and/or bottom-hugging gravity flows which disaggregated laminae and entrained discrete fragments. Particles were transported and mixed with the muddy matrix of the gravity flow. As flow energy dissipated, discrete particles (blebs) were deposited uniformly throughout the muddy matrix with their long and intermediate axes parallel to bedding; flattening during burial compaction enhanced this primary fabric. The absence of size/shape grading in SB suggests that the depositional current was viscous but was also sufficiently turbulent to permit alignment of tabular intraclasts; hence, a high-density turbidity current. Recognition of SB clarifies what were previously distinctive yet problematic deposits, and displaces erroneous interpretations of their origin by bioturbation. This study clearly defines these unique deposits, illustrates that strongly cohesive substrates are common in diatomaceous sediments, and provides criteria for their recognition in other settings.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California