DEMISE OF A DEVONIAN-CARBONIFEROUS CARBONATE RAMP BY EUTROPHICATION

The occurrence of organic-rich mudrocks overlying carbonate deposits i n the rock record has typically been ascribed to changes in relative s ea level. We propose, alternatively, that progressive eutrophication o f the euphotic zone caused the ecological demise of a Late Devonian ma rine carbonate ramp system in Alberta and its subsequent burial in org anic-rich mudrocks. Similar lithologic transitions elsewhere in the st ratigraphic record may also reflect eutrophication rather than major s hifts in sea level. The transition from Upper Devonian carbonates to D evonian-Carboniferous black mudrocks in Alberta is characterized by a gradual decrease in net carbonate accumulation as indicated by (1) an upward increase in mud, phosphate, and glauconite, (2) the existence o f firmground surfaces, and (3) a change from a dominantly phototrophic to heterotrophic benthic assemblage. An upward trend to lower 'bulk d elta(13)C(organic) and delta(15)N values reflects the eutrophication o f surface waters by upwelling of nutrient-laden deeper waters. Resulta nt primary production propelled higher organic-matter flux to the sea floor, caused the demise of carbonate-producing benthic organisms, and promoted burial of carbonate deposits by organic-rich mudrocks.