Unconsolidated late Cenozoic deposits in the northern part of the San Juan Basin range in age from late Pliocene to Holocene. Most of the deposits are alluvial gravel composed of resistant quartzite, sandstone, and igneous, metamorphic, and volcanic rocks derived from the uplifted central core of the San Juan Mountains 20–50 miles (32–80 kilometers) north of the basin. Alluvial deposits are most voluminous in the Animas Valley, but deposits of gravel of the same general age are present in the La Plata, Florida, Los Pinos, and Piedra River valleys as well. Alluvial gravel forms tabular deposits, generally about 20 feet (6 meters) thick, that are exposed beneath a sequence of terraces at many levels above the rivers. Gravel layers 360 feet (110 meters) or less above the Animas River are glacial outwash. The gravel layers begin at the south toes of end moraines and extend discontinuously downvalley at least 10–20 miles (16–32 kilometers). Farther south, distinction between outwash and nonglacial alluvium is problematical. Alluvial gravel beneath higher terraces does not grade to end moraines.

Glacial till forms a series of end moraines at the north edge of the town of Durango. The oldest moraines are farthest downvalley, are higher above the river, and have more mature surficial soils than do moraines farther north. The two youngest moraines, the Animas City moraines, are interpreted to be Pinedale in age. They have narrow, ridgelike crests and form nearly unbroken arcs across the valley floor. Small segments of still more weathered moraines, the Spring Creek moraines, are 170–230 feet (52–70 meters) above the river and are 660–990 feet (200–300 meters) farther downvalley. The oldest moraines, the Durango moraines, are on the north end of the unnamed mesa on which Fort Lewis College is located. The base is about 180 feet (55 meters) above the river. These oldest moraines may be of Bull Lake age.

Alluvial fans, pediment gravel, and landslides are scattered at several levels in various valleys within the northern San Juan Basin. Except where the Lava Creek B volcanic ash (0.639 mega-annum) is interbedded in them, these crudely bedded accumulations of sandy or clayey material washed from side drainages and added little to our reconnaissance stratigraphic study. Scattered landslide deposits consist of unsorted, mixed soil and fragments of rock.

Loess and local silty and clayey sheetwash alluvium 6–12 feet (2–4 meters) thick form a veneer on low terraces. On higher terraces, such as Red Mesa east of the La Plata River and Florida Mesa east of the Animas River, loess and sheetwash alluvium generally are about 20 feet (6 meters) thick but can be as thick as 40–50 feet (12–15 meters) in places on Florida Mesa.

When using the Lava Creek B volcanic ash as a time datum, apparently the timing of late Cenozoic continental deposition in the region was broadly similar to that in the Denver Basin–Front Range region and some other glaciated mountain ranges in the Rocky Mountains. A more accurate dating of Quaternary alluviation must await a thorough dating of the deposits. Thick, extensive outwash gravel indicates enhanced deposition during glacial epochs.

Version 1.0

Posted March 2007

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