AN ALTERNATIVE WAY TO PRODUCE BLACK SHALE RHYTHMITES: THE SIGNIFICANCE OF DEPOSITIONAL PROCESSES

Many black shale bearing successions contain intervals where highly carbonaceous black shale beds alternate with gray shales that contain little organic matter. Traditionally, this type of black shale rhythmites has been interpreted as a reflection of alternating anoxic and oxygenated conditions. Settling experiments with clays and fine grained organic matter show that the latter forms large (mm-size) flocs within minutes and clears the water column within 30 minutes, whereas clay settling under identical conditions produces much smaller flocs and much slower settling. When an organic matter/clay mixture was allowed to settle, settling curve characteristics as well as direct observation shows that a large proportion of the organic particles concentrated in the lower portion of the accumulating sediment layer, and that the upper portion is strongly dominated by clays. Implications: (1) resuspension of homogeneous bottom muds with particulate organic matter (e.g. by storms) should result in resettled (post-storm) deposits that are segregated into an organic-rich bottom layer and a clay-rich top layer; (2) the latter should enhance preservation by reducing oxygen access to the organic-rich bottom layer and void the need for bottom water anoxia; (3) sediment feeding organisms should burrow from the gray into the black layer in search of food. The proposed mechanism is dominated by sedimentological factors, rather than by surface productivity or the oxygenation state of the basin waters. It should produce black shale rhythmites that match petrographic, sedimentologic, and geochemical properties observed in black-gray rhythmites from the rock record. Prelimnary examination of examples from the rock record shows that sedimentological and geochemical features conform to the proposed model. This could have significant implications with regard to likely water depth and nature of bottom waters for a large number of black-gray shale rhythmites in the rock record.