Diagenesis of late Cenozoic diatomaceous deposits and formation of the bottom simulating reflector in the southern Bering Sea*

Abstract

Diatom ooze and diatomaceous mudstone overlie terrigenous mudstone beds at Leg 19 Deep Sea Drilling Project sites. The diatomaceous units are 300‐725 m thick but most commonly are about 600 m. Diagenesis of diatom frustules follows a predictable series of physical and chemical changes that are related primarily to temperature (depth of burial and local geothermal gradient). During the first 300‐400 m of burial frustules are fragmented and undergo mild dissolution. By 600 m dissolution of opal‐A (biogenic silica) is widespread. Silica reprecipitates abundantly as inorganic opal‐A between 600 and 700 m sub‐bottom depth. Inorganic opal‐A is rapidly transformed by crystal growth to opal‐CT. The result is formation of silica cemented mudstone and porcelanite beds. A regional acoustic reflector (called the bottom‐simulating reflector, or BSR) occurs near 600 m depth in the sections. This acoustic event marks the upper surface where silicification (cementation) is active. In Bering Sea deposits, opal‐A is transformed to opal‐CT at temperatures between 35° and 50°C. This temperature range corresponds to a sub‐bottom depth of about 600 m and is the area where silicification is most active. Thus, the BSR represents an isothermal surface; the temperature it records is that required to transform opal‐A to opal‐CT. Deposition of at least 500 m of diatomaceous sediment was required before the temperature at the base of the diatomaceous section was appropriate (35°‐50°C) for silica diagenesis to occur. Accordingly, silica diagenesis did not begin until Pleistocene time. Once silicification began, in response to sediment accumulation during the Quaternary, the diagenetic front (the BSR) moved upsection in pace with the upward migrating thermal boundary. X‐ray diffractograms and SEM photographs show three silica phases, biogenic opal‐A, inorganic opal‐A’, and opal‐CT. These have crystallite sizes of 11‐16 A, 20‐27 A, and 40‐81 A, respectively, normal to 101. The d(101) reflection of opal‐CT decreases with depth of burial at DSDP Site 192. This occurs by solid‐state ordering and requires at least 700 m of burial. Most clinoptilolite in Leg 19 cores forms from the diagenesis of siliceous debris rather than from the alteration of volcanic debris as is commonly reported.