Keller and Adatte raise four objections to the tsunami backflow hypothesis proposed by Lawton et al. (2005) for strata at the Cretaceous-Pa-leogene boundary in the La Popa basin. We summarize and address those objections in sequence below.
1. East-southeast flow directions recorded in sediment-gravity flow deposits at numerous deepwater Cretaceous/Paleogene sites along the western Gulf of Mexico record expected orientations of submarine canyons linking shelf and basinal settings; therefore, the La Popa backflow deposit is simply the upstream end of one of these canyons. This argument has no obvious bearing on the nature of the sediment-transport system that fed debris to the sediment-gravity flows. The shelf part of the sediment-bypass system must have been tied by some means to the system's deeper part and bathymetric features of the basin likely controlled the flow directions of the downdip sediment-gravity flows. However, submarine canyons probably did not extend all the way to the latest Cretaceous shoreface zone, as suggested by Keller and Adatte (2005); in the modern Gulf of Mexico, the submarine canyons terminate several tens of kilometers from the shoreline (Bryant et al., 1991). The deepwater end-Cretaceous sites of eastern Mexico actually lay in a foreland basin in which turbidity currents flowed along a foredeep from a prograding fluvial-deltaic system that lay to the west and northwest (Soegaard et al., 2003). The bathymetric profile of that basin remains poorly understood, but submarine canyons need not have been a component of the bathymetry.
2. Faunal and sedimentologic evidence in the deepwater deposits fails to tie the La Popa backwash deposits to the deepwater sites. Keller and Adatte (2005) cite the absence of coastal organisms from the deepwa-ter sites as deleterious to the backflow hypothesis. This absence is indeed puzzling, but could be explained by downcurrrent loss of larger fragments or unfortunate outcrop availability. Plant debris and Turonian mudstone clasts described from some ejecta-bearing deepwater sites (e.g., Stinnesbeck et al., 1993; Schulte et al., 2003) must have come from terrestrial or coastal settings and partially breached Laramide folds, respectively, and abundant marl clasts universally attributed to the underlying Mendez Formation may in part represent the micrite clasts in the La Popa backflow deposit (Lawton et al., 2005). There are ample observational data to tie the deepwater sites to coastal ones.
3. The deepwater ejecta-bearing deposits constitute several horizons colonized by burrowers and thus cannot represent a single event. Indeed, we expect postbackflow sediment reworking to have occurred along the early Paleogene Gulf rim. Vigorous tsunami backflow had the potential to transport and store tremendous volumes of ejecta-bearing sediment on the shelf. This ejecta-rich debris would have been available for reworking by large-scale storm events (hurricanes), a mechanism invoked by Keller and Adatte (2005), and attendant transport to deeper water within a few thousand years of the impact.
4. The deposits we attributed to supercritical flow may represent conventional deposits of current- or wave-generated processes, which are conditions intrinsic to many channels, tidal creeks, and storms. Sedimentary structures must be considered in the context of their lateral and stratigraphic arrangement for proper interpretation (e.g., Van Wagoner et al., 1990), and a vast literature exists concerning recognition of wave-, tide-, and river-dominated depositional systems. In these systems, upper flow regime conditions are transient and invariably alternate with subcritical flow conditions recorded by diagnostic sedimentary structures. The La Popa backflow deposit, with its preponderance of turbulent supercritical flow indicators, coarse grain size, substantial thickness, and absolute absence of sedimentary structures created by subcritical flow, does not conform to typical facies models for fluvial or nearshore marine environments, nor typical incised valley fills. It is, in fact, an extraordinary deposit.
In conclusion, the La Popa basin is an auspicious locality, not a poor choice, for understanding mechanisms and deposits of tsunami-induced backflow. The La Popa backflow deposit provides potential insight into how huge volumes of sediment are reworked basinward following tsunami run-up; indeed, it may force us to reexamine some existing interpretations for the origin of coeval deepwater deposits flanking the Gulf of Mexico.