Palaeoecology of graptolitic black shales

Lower Palaeozoic black shales were deposited in a variety of environments, including open oceans and shallow, near-shore areas. Some resulted from relatively rapid turbidite deposition, while others were formed slowly as a soft, anoxic ooze. Bioturbation is occasionally present, while associated paler lithologies sometimes contain dark "flakes" which were flocculated organic material or "rip-up clasts" of unconsolidated sediment. Detailed study of deep-water sequences reveals rapid lateral variation in thickness, implying an undulating sea floor in a byssal environments. Currents during the deposition of black shale are often indicated by aligned graptolites. These may also have winnowed unconsolidated graptolitic sediment to form laminae with closely packed, uniformly sized rhabdosomes. This offers an alternative to the commonly preferred explanation of "mass mortalities" which would result in a variably sized assemblage, although this can also often be established. While changes in sea level were responsible for major lithological changes, small-seale lithological alternations were probably related to fluctuations in oxygen levels controHed by current strength and density of organic material.