GRAPTOLITES: POTENTIAL INDICATORS OF HYPOXIA-ANOXIA

Most planktic graptolites occur in Ordovician - Early Devonian black shales. Analyses of probable ecologic relationships of these graptolites have led to a generally-held conclusion that most of them lived within or on the margins of hyoxic ocean waters. Certain planktic graptolites lived in relatively oxic waters near the ocean surface. These graptolites occur not only in dark shales but also in light colored, commonly bioturbated, siliclastics and carbonates deposited in shallow marine waters. The most species rich and species-diverse faunas occur in dark shales, certain of which are organic-rich and may be petroleum source rocks. Many graptolite-rich black shale successions accumulated along plate or continental margins in environments similar to those under modern oxygen-minimum zones related to upwelling. Other markedly species-rich faunas occur in dark shales, some of which are organic-rich and petroliferous, that accumulated in continental shelf environments that range from basins similar to those on the California borderland to shallow to moderate-depth shelf seas. Graptolite-bearing shales in most graptolitic stratigraphic successions commonly occur as discrete layers within or alternate with strata that either bear no graptolites or only graptolites that may be linked to oxic environments. In many shelf-sea graptolitic successions, the graptolite species present indicate alternations between hyoxic and relatively oxic conditions. In some stratigraphic sequences, graptolite taxa present in successive layers suggest gradual change from oxygen-deficient to oxic depositional environments. Discussions of the seasonal alternation of oxic to hypoxic/ anoxic environments that develop as a consequence of seasonal density stratification in the Gulf of Mexico seem to provide insights into using the presence of rich graptolite-bearing strata within stratigraphic sequences as indicators of hypoxic-anoxic conditions.