THE DEVELOPMENT OF THE PHANEROZOIC MIXED LAYER: IMPLICATIONS OF INCREASED SEAFOOD UNDER THE SEDIMENT FOR THE PRESERVATION OF EVENT BEDS

Increased extent of bioturbation across the Precambrian-Cambrian transition and through the Phanerozoic has been linked to a number of patterns seen in the stratigraphic record. Sepkoski, Bambach and Droser (1991) proposed a secular trend in Phanerozoic event beds such that preservation of thin event beds should be less common as bioturbation increases in extent and depth. Directly related to this issue is the development of the mixed layer ? that is, that part of the sediment column that is actively bioturbated by organisms. A deeper mixed layer implies that fewer thin events beds are likely to be preserved.

We have been examining the development of the mixed layer in about 15 early Paleozoic siliciclastic units representing deposition on the shelf below fairweather wave base and above maximum storm wave base based on independent sedimentological criteria. Data suggest that it is not until the Atdabanian that there is any sort of well-developed mixed layer. The very shallowest tier burrows such as Treptichnus pedum and Gyrolithes are no longer routinely preserved. However, the styles and quality of preservation are consistent with a (very) shallow mixed layer less than 5 cm. In strata of Cambro-Ordovician age exquisite preservation of relatively shallow tiers (e.g., Rusophycus and Trichophycus emplaced no deeper than 5-10 centimeters) in this setting is not uncommon. Data suggest that by the Silurian, the mixed layer may have been up to 10 centimeters deep but Rusophycus are still preserved with detailed scratch patterns. Evidence for the increased depth of the mixed layer further supports the suggestion of Sepkoski, Bambach and Droser that event beds become more thicker through this time interval and that thin beds are rarely preserved. A consistent mixed layer of 5 centimeters should preclude the common preservation of beds less than that in thickness. The best preservation of thin event beds in this setting would be the Precambrian and the very earliest Cambrian.