ABSTRACT

The basic building blocks of clay fabric in recent muds from Santa Monica Basin and closely analogous mudstones from the Los Angeles and Ventura basins are floes 2-200 µm in diameter. "Physico-chemical floes" are thought to result from the electrical attraction of clay particles and have an open fabric of clays in dominantly edge/face (EF) clay particle contacts. Such floes are roughly equidimensional, but boundaries between them are generally poorly defined. "Bioflocs" are thought to result from biological agglomeration during ingestion by benthic and pelagic organisms. These are more densely-packed flocs containing mostly face/face (FF) or low-angle EF clay particle contacts; organism tests are a common constituent. Bioflocs are equidimensional, usually ovoid, the individual f oes are well-defined.

In this study, bioturbated sediments near the sediment/water interface contained abundant bioflocs, silts, micas, and tests. With increasing burial depth (275 m), these bioflocs became squeezed between silt particles and formed welded packages of clays exhibiting abundant clay faces. At burial depths of 640 m, consolidation and diagenesis produced crenulate and intergrown clay structures.

Turbidite muds near the sediment/water interface were dominantly composed of physico-chemical flocs. As burial depth increased, high-angle EF clay particle contacts changed to low-angle contacts, and silt particles began to influence the fabric of nearby clays. At burial depths of 275 m, low-angle EF contacts altered to very low-angle EF and FF contacts. Some preferred orientation of clays developed in thin laminae, wrapped around silts in adjacent laminae. At burial depths of 640 m, diagenesis and consolidation produced densely welded zones of crenulate, intergrown clays virtually indistinguishable from nearby bioturbated mudstones.

Pelagic muds had a more heterogeneous fabric, containing both bioflocs and physico-chemical floes in various proportions, along with abundant organism tests. With increasing burial depth, bioflocs became more densely welded and intergrown with other fabric features; individual packages were difficult to discern. The physico-chemical floes reoriented to moderately well-developed preferred orientation with very low angle EF and FF particle contacts. Diagenesis created a crenulate, intergrown clay fabric.