Speckled beds; distinctive gravity-flow deposits in finely laminated diatomaceous sediments, Miocene Monterey Formation, California

Abstract

Distinctive nonlaminated intervals we term speckled beds (SBs) are common in finely laminated diatomites of the Monterey Formation at Celite Quarry, Lompoc, California. SBs have sharp and bedding-parallel upper and lower contacts, tabular geometry, and are less than or equal to 10 cm thick. SBs are characterized by a bedding-parallel arrangement of sand- to granule-size diatomaceous and detrital aggregates that are uniformly distributed in a fine-grained matrix of macerated biosilica and detrital mud. The unique physical properties of diatomaceous sediments provided cohesive substrates and high fluid content that were prerequisite for the formation of SBs. Abundant hybrid fault/vein structures, slump folds, and complexly deformed laminae in the quarry attest to syndepositional deformation of laminated diatomaceous sediments on a subaqueous slope within the oxygen-minimum zone. The unique features of SBs permit an interpretation of stepwise rheological transformation during mass movement, via passage through critical-point transformations, from Bingham plastic (slump and/or slide block) to turbulent fluid (turbidity current) to viscous fluid exhibiting laminar flow (debris flow). The recognition of SBs clarifies the origin of these distinctive yet problematic deposits, and clearly differentiates them from other nonlaminated intervals. SB composition indicates provenance from an oxygen-minimum zone and further illustrates that a hiatus is not required to form cohesive firmgrounds in diatomaceous sediments. The recognition of SBs as recorders of slope failure events complements paleoseismic studies of Holocene diatomaceous laminites from Saanich Inlet, British Columbia (Ocean Drilling Program Sites 1033 and 1034).