Orbital-driven cyclicity and the role of halokinesis on accommodation within siliciclastic to carbonate, shallow-water Albian deposits in the Espírito Santo Basin, southeastern Brazil

Abstract

The basal section of the lower Albian Barra Nova Group in the onshore portion of the Regência Platform, a central-eastern geological province of the Espírito Santo Basin, Brazil, comprises a mixed succession characterized by the cyclic alternation of dominant fluvial to shallow marine, siliciclastic rocks and marine carbonate strata. Spectral analysis of gamma-ray logs of four wells has shown consistent evidence of a short eccentricity orbital frequency (100 ka) influence on sedimentation. The spectral analysis points to a time span from 2.95 to 3 Ma (depending on the method employed) for the deposition of the interval studied in the most complete section (Well 1), and from 1.84 to 2.1 Ma in the least complete section (Well 4), owing to partial erosion. These figures are consistent with the palynological dating of the section (2 Ma), hence showing that cyclostratigraphy can play an important role in assigning a time span for high-frequency cycles (4th and 5th orders). Climatic factors not necessarily linked to eustasy, such as the alternation of dry (carbonate deposition) and mild humid (increased siliciclastic input) climate, are supposed to have controlled the cyclical stratigraphic arrangement. Accommodation as a result of diastrophism, loading and eustatic sea-level rise would not explain such a high mean accumulation rate (29.1 cm/ka) estimated through cyclostratigraphy. Therefore, based on these assumptions and seismic data, the high preservation potential, and resulting well-preserved cyclicity, is herein assigned to halokinesis. Several pieces of evidence obtained from the analysis of seismic data, core description, stratigraphic correlation, palaeontology and spectral data have shown the importance of halokinesis to increasing accommodation, and then the excellent preservation of the cyclical nature of the geological record. This is unusual in sedimentary environments subject to recurrent erosive events such as those related to shallow-marine settings.