Phosphate Maximum Zone

Karl B. FOllmi

The term "phosphate maximum zone" defines marine environments in which biochemical and physical conditions favor phosphogenesis, as well as subsequent accumulation of phosphatized components into conglomeratic, "condensed" beds.

Data evaluated from phosphatic condensed beds within the Aptian-Albian Garschella Formation (Austria) and comparative studies of similar beds within the Miocene Monterey Formation and the Pliocene Sisquoc and Purisima Formations (California) suggest the following constraints on the evolution of "phosphate maximum zones."

1. Advantageous physical conditions will be provided by major currents that act to lower accumulation rates and to winnow and redistribute sediments.

2. Major current activity will limit the upward expansion of oxygen minimum zones. Within the transition zone at the top of the oxygen minimum zone, optimal biochemical conditions will consist of favorable Eh/pH values, high preservation rates of organic material, and sufficient supply of nutrients (current-dependent).

The presence of low accumulation rates and optimal biochemical conditions within the upper boundary zone of the oxygen minimum zone may result in sufficient influx and biochemical concentration of phosphate, calcium, and fluorine in pore waters of the uppermost sediment layer and, subsequently, in phosphogenesis.

Variable current velocities ("water jets") and/or lateral shifts in current direction, induced by sea level changes, cause changes in accumulation rates, and may result in erosional events. Therefore, phosphogenesis can occur repeatedly at the same stratigraphic level, resulting in different phosphate generations. Furthermore, phosphatic nodules can be concentrated into conglomeratic "condensed" beds during increased current activity.

Extreme current pulses can transport phosphatic nodules long distances, leading to the accumulation of conglomeratic, phosphatic beds in variable environments outside the phosphate maximum zone.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.