Abstract: Depositional Significance of Lacustrine Black Shales: Falsifying a Paradigm

Guzzo, J. V. P. and Arienti, L. M. - Petrobras/Cenpes

Limnological studies generally emphasize numerous physical, chemical and biological aspects of lake waters, although frequently with little concern about possible sedimentary responses to specific limnic phenomena. Due to the lack of adequate actualistic lacustrine facies models, paleoenvironmental analyses of pre-Quaternary lacustrine strata are usually based on sedimentological analogies with marine examples.

Lacustrine black shales have been usually interpreted as transgressive deposits, much like marine condensed sections. Two case studies from Brazilian Lower Cretaceous rift basins suggest that this direct analogy can be misleading.

A detailed sedimentological study based on outcrop and well descriptions of the Maceio Formation (Sergipe/Alagoas Basin) reveals a recurring fining-upward succession of sandy turbidites and gray shales overlain by black shales with exposure features (mud-cracks and algal-mat lamination) (Fig. 1). In another comparable example, an integrated approach (sedimentological, geochemical and paleontological) of deltaic-lacustrine deposits of the Pojuca Formation (Reconcavo Basin) shows a gradual oxygen isotopic enrichment (d¹⁸O_carb) toward black shale intervals (Fig. 2). For such a tropical paleolake, it is also strong evidence of decreased water volume during black shale deposition. In both studies, the cyclicity of the stratigraphic data delineates climatically induced lacustrine cyclothems. Climatic excursions towards arid conditions appear to have simultaneously lowered lake-level, sediment supply and bottom oxygen levels, thus favoring the organic accumulation.

Evidences of salinization (e.g., high d¹⁸O values, low C/S ratio, dolomitization, evaporite crystals or pseudomorphs) or even exposure (mud-cracks), as exemplified by the two above mentioned cases, are not rare among lacustrine black shale descriptions. These evidences suggest that, contrary to prevailing belief, black shales could be the representative lithofacies of lowstand lake-level episodes in detrital lakes.

A number of artifices have been used to avoid the paradoxes that arise when the classical view of lacustrine source rock formation (deposition below storm wave base-level in deep anoxic lakes) is confronted with stratigraphic evidence of low water depth and increased salinity. Diagenesis is frequently used to explain such anomalies. Sometimes marine incursions into lake basins are claimed. Organic-rich facies are also generally referred to as being concentrated in basin depocenters, and not overlying marginal lacustrine facies, which would be expected for a transgressive unit.

In addition to variations in the rate and nature of the sediment supply to lake basins, we consider that variable rates of organic matter degradation and nutrient recycling in the water column, affected by climatically induced changes in the size, salinity and thermocline depth of lakes, should also be taken into account to accommodate the previous data and arguments in formulating a model concerning source rock formation in lacustrine sequences

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil