Biostratigraphy

As noted in chapter 2, this excavation included five strati-graphic levels, the sediments of which very closely resembled the distinctive loose, dry dust of levels 1-3 of the Pit. Paleo-magnetic samples taken from beneath the lowest level were interpreted as questionably normal by Fred Luiszer and V. A.

FIGURE 7.4 Correlation of the Pit sequence with the CM and DMNH Velvet Room excavations. Correlation of the sections is based on the biostratigraphic zones defined in the Pit and the local magnetostratigraphic evidence in each section. Correlation to the time scale is by biochronology and assumption that the reversed (white) magnetic interval indicates the Matuyama magnetic epoch. Dark gray shading denotes parts of the respective sections that fall in the Mictomys Zone. Climatic intervals are defined by sediment type. Numbers in the Pit Sequence column schematically represent the stratigraphic levels. Heavy dotted lines denote suspected unconformities. N in the Velvet Room columns indicates normally magnetized (black) parts of the section. Question marks in the Pit Paleomag. column indicate lack of data. The question mark at the base of the DMNH Velvet Room column expresses uncertainty about the age of the basal part of the section.

Schmidt (pers. comm., 1993). Most of the fossils have not yet been processed, but based on the small sample of arvicolines so far identified (see table 10.11), the deposits seem to fall within the Microtus 5T Relative Abundance Zone. The lowest fossiliferous level in the excavation, level 3, is the only one from which a reasonably large sample of arvicolines has been identified, and it shows the following percentages for mini mum number of individuals: Mictomys, 19%; M. meadensis, 19%; and Microtus 5T, 23%.

Of interest is one specimen of AHophaiomys pliocaenicus present in level 3. This finding hints that level 3 of the CM Velvet Room could fall within the Allophaiomys Assemblage Zone of the Pit, although the other requisite taxon, Mimomys virginanus, has not been recovered from the CM Velvet Room. The presence of A. pliocaenicus suggests that level 3 in the CM Velvet Room could be as old as level 4 in the Pit.

However, all other lines of evidence suggest that the CM Velvet Room deposits are younger than the Allophaiomys Assemblage Zone sensu stricto (i.e., also containing Mimomys virginianus) in the Pit. In the Lemmiscus sample from level 3 in the CM Velvet Room, approximately 31% of the specimens are four-triangle morphs; the rest are five-triangle forms. In the highest level of the CM Velvet Room, 100% of the Lemmiscus specimens exhibit either five or six triangles; six-triangle forms also occur in level 2A. This finding would suggest that levels 1 and 2 in the Velvet Room are younger than any levels in the Pit, possibly excepting Pit level 1 (see figure 23.8).

These lines of evidence suggest that the CM Velvet Room excavation is at least as young as levels 1-4 in the Pit. The presence of six-triangle forms of Lemmiscus in CM Velvet Room level 2A implies correlation with level 1 (or younger) in the Pit. The high percentage of five-triangle morphs of Lemmiscus in CM Velvet Room level 3, combined with the clear dominance of Microtus 5T and M. meadensis over Mictomys, suggests that even that level is younger than Pit level 4, despite the occurrence of Allophaiomys pliocaenicus. The equivocally normal paleomagnetic signature at the base of the excavation hints that the sediments in the CM Velvet Room excavation are younger than 780 Ka, but the paleomagnetic data are far from firm.

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