LIMESTONE FORMATION IN THE ORGANIC-RICH MARLS OF THE CENOMANIAN-TURONIAN EAGLE FORD GROUP OF SOUTH TEXAS

The origin of limestone/marl couplets has been the subject of numerous studies, with no clear agreement as to the primary factors controlling limestone deposition in organic-rich shales. Some researchers have postulated that the limestones and marls were deposited as undifferentiated mudstones, with suggested mechanisms for secondary limestone formation including dissolution of aragonite and reprecipitation within adjacent layers, and precipitation of calcite cements during periods of non-deposition. Hypotheses for a primary origin of limestone/marl couplets include dissolution, dilution, productivity, and redox cycles.

Limestones are common within the organic-rich marls of the Eagle Ford. Examination of cores from the subsurface of south Texas found that most limestones contain high abundances of calcispheres and calcified radiolaria, with lesser abundances of calcite-filled planktonic foraminifera. Intact fecal pellets and foraminifera tests suggest calcite precipitation prior to significant compaction. Bottom-water oxygen levels, as indicated by bioturbation and geochemical redox proxies, were higher during limestone deposition than during deposition of the adjacent marls. The marls are laminated, contain common to abundant kaolinite-filled planktonic foraminifera, no calcispheres, and very rare pyritized radiolaria.

The Eagle Ford limestones are hypothesized to have formed during periods of high fertility and enhanced water-column mixing. The latter increased bottom-water oxygenation, whereas possible upwelling along relict reef margins provided nutrients to the surface waters and produced blooms of radiolaria and calcisphere-producing dinoflagellates. Alkaline conditions near the sediment–water interface due to oxidation of organic matter were conducive to calcite precipitation, initially filling foraminifera tests, calcifying pyritized radiolaria, and cementing lag deposits, and then progressing to intra-particle calcite precipitation.