Climate and Orogeny

The difference in wood rat species composition between the time represented through the Pit sequence and the present suggests major biogeographic changes in the Porcupine Cave area. Three causes seem possible: orogenic elevation of the South Park area since the Pit fauna was deposited, causing the loss of a Great Plains-like habitat that once had been continuous with South Park; climate change; or a combination of the two.

The climate change that would permit wood rats, now living on the Great Plains of southeastern Colorado below 1830 m elevation, to live near Porcupine Cave would seem to include increased warmth and dryness. The paleomagnetic correlations detailed in chapter 7 indicate a time equivalence to the mid-"Nebraskan" glacial advances across the northeastern Great Plains. Although this would result in warmer winters, it would also result in cooler summers with more moisture.

Tectonic uplift in the Colorado Rocky Mountains in the last 780,000 years would be consistent with the occurrence of Neotoma species in the Pit fauna, and it is consistent with correlative glacial history in the nearby Upper Arkansas Graben. This graben lies 16 km west of Porcupine Cave and is part of the Rio Grande Rift that runs south from Leadville to Salida, Colorado (see figure 4.1). At Salida the Upper Arkansas Graben is separated from the remainder of the Rio Grande Rift by a northwest-trending transference from the Sangre de Cristo Mountains. Poncha Pass carries highway traffic across this uplifted transference and into the San Luis Valley, now continuous, as the Rio Grande Rift, all the way to Mexico. Since the Upper Arkansas Graben began sinking as a result of tension caused by uplift of the Colorado Plateau, some 7 Ma ago, deposits have accumulated in it, leaving a record of the history of regional uplift and glaciation. The Wagon Tongue Formation that overlies the Rocky Mountain Peneplain (Lee, 1923), forming most of South Park (De Voto, 1971), contains vertebrate fossils that are about 15 million years old and predate the Upper Arkansas Graben.

There are three points of time control within the graben: the circa 7-million-year-old fossils found in the lake or marsh beds near Salida, which were the first deposits in the developing graben; the 780,000-year-old Bishop Ash, which was deposited in the graben on the floodplain of the ancestral Arkansas River (it is also deposited on river floodplains in the San Luis Valley; Rogers et al., 1985, 1992); and the 670,000-year-old Lava Creek ash between glacial outwash gravels. The Bishop Ash has been used to date the beginning of the Brun-hes normal-polarity interval (Obradovich and Izett, 1992) and thus provides a minimum age for the reversed parts of the Pit sequence, that is, levels 8 and below. It also was deposited during the "Nebraskan" (glaciations E [normal], F [reversed], and G [reversed] of Richmond, 1986).

Although the "Nebraskan" featured the most extensive continental ice sheet that covered the Great Plains, no indication of mountain glaciation during this time has been found in the Upper Arkansas Graben (Richmond, 1986). Instead, the oldest mountain glaciation recorded in the Upper Arkansas Graben is "Kansan" glaciation represented by deposits that straddle the Lava Creek Ash, dating from about 670 Ka.

Mountain glaciation occurs in mountains because of cooler and wetter climates at higher elevation, and one possibility is that the Upper Arkansas Graben, 16 km west of Porcupine Cave, and the areas surrounding it were at a lower elevation when the Pit fauna lived in that area. This hypothesis would be consistent with the elevation indicated by the unusual assemblage of wood rat species. The other possibility is that elevations were the same as today, but that the glacial climates of the earlier part of the "Nebraskan" were not as cool and moist in South Park as those coeval with the Lava Creek Ash. Because chapters 7 and 23 discuss climatic scenarios in more detail, the remainder of this discussion focuses on possible tectonic interpretations.

Today, about 24 km west of the Upper Arkansas Graben, the Continental Divide, averaging between 3650 and 3960 m in elevation with many peaks higher than 4370 m, drains into the graben (see figure 4.1). These mountains were clearly glaciated during the later glacial episodes, but there is no indication that this was so when the wood rats of the Pit fauna lived. This too would be consistent with lower elevations during the "Nebraskan" glaciations.

The modern route of the Arkansas River passes southward down the Upper Arkansas Graben to the west of South Park and Porcupine Cave, and then south of them toward the Great Plains at Canon City (see figure 4.1). Because of its depth of incision, the Arkansas River is now capturing, through Badger Creek, the drainage of some of the southernmost part of South Park. The immediate area of Porcupine Cave is drained by Badger Creek today. However, the cave is not far removed from the modern headwaters of Cottonwood Creek, which flows southeastward from South Park to the Arkansas River near Canon City. Recent uplift may have enabled Badger Creek to capture the drainage of this part of South Park, and the Arkansas River is deeply incised between Canon City and Salida. These geomorphic features are also consistent with uplift of the Porcupine Cave region since the Pit fauna lived.

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