Paper No. 173-9
Presentation Time: 10:05 AM
MILANKOVITCH CLIMATIC SIGNALS AND STRATIGRAPHIC COMPLETENESS OF THE ERNST TINAJA SECTION OF THE UPPER CRETACEOUS DEEP-MARINE EAGLE FORD FORMATION, BIG BEND NATIONAL PARK, TEXAS
The Ernst Tinaja section is multi-order cyclic and composed of dm-m-thick alternating hydrodynamically-recycled and pelagic facies associations deposited on the deep-water shelf of ancestral Gulf of Mexico. The section contains 211 cycles and is 109 m thick. Its periodicity is assessed through gamma and spectral analyses. A decompaction factor of 1.74 is used to restore original thickness of mudrocks and carbonates. Gamma analysis (Kominz & Bond 1990) assumes that sedimentation rate is facies dependent and is an over-constrained linear regression method that estimates the sedimentation rates of individual facies for conversion of the facies curve in thickness into a time series. The time series is then subject to spectral analysis. The gamma results show consistent and clearly-separated rates for 3 combined facies: limestone, chalk, and mudrock. The spectrum has 7 statistically significant peaks. Their periods match with periods of long and short eccentricity (405-270, 127-94 k.y.), long and short precessional index (PI, 23-22, 19-18 k.y.), long and short obliquity (53-49, 40-35 k.y.), and the constructional tone between long and short PI cycles (11.8 k.y.). The match suggests that Milankovitch orbitally-induced climatic oscillations significantly influenced sedimentation. The sedimentation rates of the 3 facies are 2.1, 3.3 and 12 mm/k.y. for limestone, chalk and mudrock, respectively, based on the period calibration. The higher rate of mudrock indicates continuous deposition and good preservation, whereas the lower rate of limestone episodic deposition, reworking and erosion. Finally, the depositional duration represented by rocks, excluding that of missing cycles, was calculated as 2.02 m.y. The ratio between depositional duration and total Eagle Ford duration (7.3 m.y.) is 28%, indicating a low stratigraphic completeness. This results from abundant depositional hiatuses and erosional gaps at the bed scale and at member boundaries within and bounding the formation. The interaction of long-term sea-level fluctuation and Milankovitch forcing of cyclicity, and high stratigraphic incompleteness provide insight into formation of deep-marine epicratonic mudrock-carbonate successions.