“ANÁLISE CICLOESTRATIGRÁFICA DOS DEPÓSITOS SEDIMENTARES DA BACIA POTIGUAR DURANTE O INTERVALO CENOMANIANO-TURONIANO”

Abstract

Exploratory sedimentary basins require continuous chronostratigraphic refinement. However, in continental settings, this remains challenging due to the complex and dynamic erosional processes of fluvial and fluvio-estuarine systems, which often obscure the depositional record. Consequently, investigating the influence of orbital cycles - an approach that can aid in refining chronostratigraphic frameworks - in these records is also a challenge. As a result, studies on this topic are relatively uncommon in the literature, despite the growing recent interest from the scientific community. In this context, the fluvio-estuarine deposits of the Açu Formation, in the Potiguar Basin - an important exploratory basin located on the Brazilian equatorial margin - represent an special sedimentary archive in which T-R cycles can be traced laterally for tens of kilometers. This makes them an interesting subject for cyclostratigraphic analysis in non-marine environments. This study investigates the influence of orbital forcing on the Açu Formation, correlate cycles influenced by orbital variations in the wells, and establish the preserved time interval through the construction of an Astronomical Time Scale (ATS). Cyclostratigraphic analyses were performed using time series techniques on gamma-ray logging data from three wells, focusing on two third-order sequences deposited from the Cenomanian-Turonian boundary (approximately 101 to 93 million years ago). Statistical methods including correlation coefficient (COCO) and the null hypothesis (H0), were applied to determine the ideal sedimentation rate (SAR) and test the record of astronomical cycles. The Integrated Error Prediction Filter Analysis (INPEFA) was used to identify trends in the sedimentary record, improving the correlation of matching cycles. The cyclostratigraphic results demonstrate the imprint of orbital forcing on the sedimentary deposits, consistent with the astronomical solutions for the Cenomanian-Turonian interval. The inferred sedimentation rates range from approximately 3 to 8 cm/kyr, consistent with similar depositional environments. The identification of spectral bands corresponding to the long eccentricity cycle (405 ka) allowed for the construction of a floating ATS for the Cenomanian-Turonian interval in the basin and the estimation of the recorded time. Since the good preservation of orbital signals in fluvio-estuarine environments is uncommon - limiting the number of cycle-stratigraphic studies - this research provides important insights into the behavior of orbital signals and basin filling in this type of paleoenvironment. Furthermore, as this interval includes hydrocarbon-bearing deposits, the findings contribute to the advancement of cyclostratigraphic studies in exploratory basins, with potential applications for chronostratigraphic correlation.