Models for Formation of Organic-Rich Diatomaceous Laminae in Miocene Monterey Formation: Documentation from Analogous Recent Sediments in Gulf of California: ABSTRACT

Abstract

Depositional modeling of the Monterey Formation has traditionally focused on basin scale models. Analysis of homogeneous and laminated, organic- and diatom-rich sediments from the central Gulf of California permits modeling of small-scale depositional environments that may be useful in mapping within the Monterey Formation. Sedimentologic, micropaleontologic, and geochemical characteristics of recent Gulf of California sediments permit definition of three depositional environments. (1) Thick (~ 1 cm) lamination of the silled anoxic San Pedro Martir Basin. The laminae are indistinct in color and contain a uniformly mixed opal phytoplankton population of both oceanic and upwelling floras. Only the darker laminae contain the silt- and clay-sized material washed into the area during the rainy season. (2) Thin (~ 1 mm) lamination of slope sediments where the oxygen minimum zone impinges the sea floor (~ 400 to 1,300 m water depth) on both the Baja and mainland sides of the Guaymas Basin Slope. Laminae are generally distinct in color and consist of alternating oceanic and upwelling opal phytoplankton assemblages. Dark laminae contain clay- and silt-sized terrigenous material. Dark laminae on the Baja side also contain an upwelling microfloral assemblage. Dark laminae on the mainland side contain an oceanic microfloral assemblage. (3) Homogeneous sediments overlie laminated sediments off Santa Rosalia. The homogeneous sediments are characterized by uniformly distributed microfloral and macrofloral elements and terrigeneous material due to bioturbation by a mollusk infauna. The differences between the various laminated and the nonlaminated (= homogeneous-bioturbated) facies are due to differing levels of primary productivity in surface waters which in turn controls oxygen consumption. Oxygenated bottom water underlies low primary productivity areas while anoxic bottom water underlies high primary productivity areas. End_of_Article - Last_Page 891------------