Stinnesbeck et al. address some important uncertainties about the sedimentologic and sequence stratigraphic origins of the tsunami deposit in the La Popa basin. We welcome the opportunity to expand upon these aspects of the deposit.
Stinnesbeck et al. mistakenly understood us to argue for tsunami run-up on the basis of “deeper water elements in a mostly shallow-water assemblage.” In fact, we argue the opposite: The gastropods, oysters, bryozoans, foraminifera, and even ammonites represent a shallow marine to coastal faunal assemblage mixed and dragged to deeper water by the return flow. Micrite and calcareous siltstone fill the gastropods and are present as intraclasts, which are locally armored by ejecta grains. These lithic types are not known elsewhere in the uppermost Cretaceous basin stratigraphy (e.g., Aschoff, 2003); they were likely derived from coastal environments such as estuaries and salt marshes that lay beyond the basin. The mixed shallow marine to coastal fauna lying offshore of its original habitat is a key argument for the polarity of the flow. We regret if this point was not clear in the paper.
As noted by Stinnesbeck et al., the precise age of the valley-fill deposit is imperfectly constrained. The ammonite, Sphenodiscus pleurisepta, is present up to and in the valley-fill deposit. Five km to the south, the Paleocene nautiloid, Cimomia haltomi, is present in what is probably a transgressive lag at the base of the overlying upper mudstone member of the Potrerillos Formation (Vega et al., 1989; Lawton et al., 2001). Micrite fill of ammonites in the valley fill indicates that they are reworked. Thus, the valley-fill deposit lies at or near the stratigraphic Cretaceous-Paleogene boundary. Our interpretation of a Cretaceous-Pa-leogene unconformity (Lawton et al., 2001) predated recognition of both Chicxulub-derived ejecta in the valley-fill deposit (Aschoff et al., 2001) and obvious differences in the sedimentology of the valley fill and near-diapir debris flow deposits, present only within 1 km of the El Papalote diapir (Rowan et al., 2003). On the basis of biostratigraphy alone, one may conclude that the valley-fill deposit does not precisely record the Mesozoic-Cenozoic boundary, and therefore might not be coeval with an end-Cretaceous impact. However, stratigraphic restriction of the ejecta to a single level incised into Upper Cretaceous strata, intimate association of ejecta and weakly lithified intraclasts indicated by ejecta armoring, and a mixed fauna with fill resembling the intraclasts combine to indicate that ejecta arrival and the large erosive event were nearly simultaneous impact-related phenomena.
A general lack of diapir-derived detritus in the valley-fill deposit makes the diapiric-source hypothesis offered by Stinnesbeck et al. (2005) untenable. The nearby El Papalote diapir was a unique source for mafic metaigneous clasts present in nearby Upper Cretaceous through Paleo-gene strata (Garrison and McMillan, 1999; Giles and Lawton, 2002). Although locally abundant in strata flanking the diapir (Lawton et al., 2001; Giles and Lawton, 2002; Rowan et al., 2003), metaigneous clasts are extremely rare in the valley fill. Moreover, red algal detritus derived from diapir-flank bioherms (e.g., Hunnicutt, 1998) is a common component of strata near the diapirs, but is rare in the valley fill. Although some debris was eroded from the diapir by the back flow, and a small amount of that material resides in the valley fill, the tsunami deposit had a fundamentally different provenance than the strata derived from the diapirs.
The sequence stratigraphic position of the valley-fill deposit is critical to understanding its origin. A basinwide reappraisal of the Delgado Sandstone Member of the Potrerillos Formation (Aschoff, 2003) revealed that the ejecta-bearing beds do not represent a conventional valley fill overlying a sequence boundary. Rather, the valley fill lies within a retrogradational parasequence set in the upper part of the Delgado Member and overlying upper mudstone member, and therefore occupies a transgressive systems tract. The tsunami deposits thus punctuate the sequence stratigraphy of the basin, occupying a stratigraphic position not predicted by base-level models, an observation that emphasizes their allogenic event origin.