Toward a Sequence Stratigraphy of the Chicxulub Impact Basin Infill
Michael T. Whalen and Zulmacristina F. Pearson University of Alaska Fairbanks
Sean P. Gulick University of Texas at Austin
Co-PIs Michael Whalen and Sean Gulick were funded in 2005 under a pre-drilling activity proposal to conduct stratigraphic analysis of the Yaxcopoil-1 (Yax-1) core and investigate the relationship of the core to offshore 2D seismic data to provide insight into the Tertiary infilling history of the Chicxulub impact basin. High-resolution logging of approximately 400 m of Tertiary carbonate-dominated sedimentary rocks in Yax-1 (Dressler et al., 2003) provides details of the lithostratigraphy and biostratigraphy and permits a preliminary sequence stratigraphic analysis of the post-impact succession in the basin. Approximately 290 samples were collected for petrographic, x-ray diffraction and biostratigraphic analyses. Analysis of offshore 2D seismic data provides a broader view of the process of Tertiary basin infilling and the history of Yucatàn carbonate platform development.
Figure 1. Interpreted seismic line from the northeast portion of the crater illustrating the position of the K/T boundary and six overlying seismic units. Note the two sets of prograding clinoforms in seismic Unit C. Inset is a photograph from sequence 3, ~640 m Yax-1 core, illustrating graded turbidites that may be similar to the deposits making up toes of prograding clinoforms in seismic Unit C. Modified from Pearson et al. (2006).
Ten lithofacies were categorized into two broad groups: redeposited and background facies that occur in 5 lithostratigraphic units. Redeposited facies include carbonate supportstones of a wide array of grain sizes and finer-grained facies with evidence of soft sediment deformation, all of which were deposited by a variety of gravity flow mechanisms. Background facies include fine-grained argillaceous and clean limestones that were deposited mainly from suspension, below storm wave base, at depths ranging from bathyal to neritic. Depositional environments range from a deep-water, steep slope inside the Chicxulub crater rim to outer carbonate ramp neritic environments that were established once the Yucatàn platform had prograded seaward.
Five depositional sequences were identified based on transgressive and maximum flooding surfaces and facies stacking patterns. Biostratigraphic data remains equivocal but indicate that the first 3 sequences range from Early Paleocene to Early Eocene in age. The base of sequences 1 through 4 contain coarse-grained redeposited carbonates interpreted as lowstand gravity-flow deposits, while the highstand of sequence 3 records the first evidence of relatively fine-grained turbidite deposits that we interpret as the initial record of Yucatàn platform progradation into the Chicxulub basin (Fig. 1). Sequences 4 and 5 consist mainly of background and fine-grained redeposited facies but by the top of sequence 4 indicate that the Yucatàn platform slope and outer ramp had prograded over the position of the Yax-1 core. The upper portion of sequence 3 and lower sequence 4 appear to straddle the Paleocene-Eocene boundary. A series of soft sediment deformed units in sequence 3 may have been deposited in response to events associated with the Paleocene-Eocene thermal maximum; however, better biostratigraphic control will be necessary to test this hypothesis.
Six seismic units were identified, the lower 5 of which appear to roughly correlate with the 5 sequences in the Yax-1 core (Fig. 1). The geometry and distribution of seismic units A and B are strongly controlled by the morphology of the crater. Unit C, with two sets of clinoforms, records a major progradational event in the eastern portion of the basin that may to be related to turbidite deposition in sequence 3 in Yax-1 (Fig. 1). The timing of significant platform progradation into the basin near Yax-1; however, appears to be during sequence 4, i.e. later than in the eastern part of the basin. Seismic units D and E display parallel reflectors indicating relatively level bottom conditions similar to the depositional environments indicated by lithofacies in upper sequence 4 and sequence 5. By the time of deposition of unit E, the western and central parts of the basin were mostly infilled. The top of units E and D, at the edges of the inner crater, is an erosional truncation surface that marks the base of unit F. Unit F, overlies an erosional truncation that cuts into units D and E, and is characterized by discontinuous reflectors that are restricted to the northeastern portion of the basin, the last part of the basin to be infilled during the Tertiary (Pearson et al., 2006; Whalen et al., 2007; Gulick et al., 2008).
Figure 2. DOSECC’s Hybrid Coring System installed on a rotary rig, drilling the Yax-1 core.
Chicxulub is the largest well-preserved impact structure on Earth. Analysis of the Yax-1 core and comparison with patterns of deposition revealed with 2D seismic data provide us with information about the post-impact history of Chicxulub and the history of infilling the impact basin. If Chicxulub is representative, large marine impacts in tectonically quiescent regions may dominate local depositional environments for millions to tens of millions of years post-impact before returning control to eustasy. Successions within impact basins will likely record shoaling and slope readjustment as the instantaneously created basin is filled on geologic timescales. Our understanding of the processes and consequences of large-scale impacts have been enhanced through study of the Chicxulub impact structure and sediment infill. The relatively rapid, post-impact recovery of the Yucatàn carbonate platform demonstrates the resiliency of the carbonate-producing systems to even the most catastrophic, mass-extinction inducing events.
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Gulick, S. P. S., P. J. Barton, G. L. Christeson, J. V. Morgan, M. McDonald, K. Mendoza-Cervantes, Z. F. Pearson, A. Surendra, J. Urrutia-Fucugauchi, P. M. Vermeesch, and M. R. Warner (2008), Importance of pre-impact crustal structure for the asymmetry of the Chicxulub impact crater, Nature Geoscience, v. 1, p. 131-135.
Pearson, Z.F., Whalen, M.T., Gulick, S.P., and Norris, R., 2006, Annealing The Chicxulub Impact: Tertiary Yucatan Carbonate Platform Development and Basin Infilling, Geological Society of America Abstracts with Programs, v. 38, p. 297.
Whalen, M.T., Pearson, Z.F., and Gulick, S.S.P., 2007, Toward a Sequence Stratigraphy of the Chicxulub Impact Basin Infill: Integration of Lithostratigraphy, Biostratigraphy, and Seismic Stratigraphy: Final Report of Pre-drilling Activity Proposal to the Joint Oceanographic Institutions, 27 p.