New Mexico Geological Society Annual Spring Meeting — Abstracts

The floodplain architecture of the Pecos River valley within the Bitter Lake quadrangle is comprised of: 1) basal late Pleistocene braided stream deposits; 2) distinct Holocene (2) and historic (2) meanderbelts; 3) backswamp or spring cienega deposits beneath older Pecos River terraces to the west; 4) thin alluvial veneers and karstic collapse depressions atop of Permian Seven Rivers Formation gypsum on the east; 5) a large, late Pleistocene braided fluviodeltaic fan complex of the Rio Hondo to the southwest; and 6) 2 early to middle Holocene Rio Hondo meanderbelts that prograded across the Pecos River floodplain below the confluence. Pecos River alluvium is light reddish brown, medium-grained quartzose sand and pebbly sand; gravel consists of sandstone, quartzite, chert, igneous rocks, and minor carbonates, except in backswamp areas, which contain silt, clay, and gypsite. South of the Rio Hondo confluence, Pecos River alluvium intermixes with Rio Hondo alluvium, which is dark yellowish brown in color, and contains abundant porphyritic igneous and carbonate gravel. Floodplain alluvium thickness decreases from a maximum of around 45 m along the western margin to <1 m to the east.

Large Pleistocene-aged braid plains are exposed on the northern and southern edges of the quad. These resulted from steeper gradients, easily erodible banks, and a much greater variability in discharge. At the end of the Pleistocene, vast quantities of sediment-rich, glacial meltwater flowing out of the Sacramento Mountains down the Rio Hondo built a vast fluviodeltaic fan out onto the Pecos River floodplain. This initiated meandering atop of the braid plain upstream of the fan in response to a decrease in gradient. A segment of this initial meanderbelt, preserved north of Bitter Lake, likely carried flows concomitantly with braided channels to the east during the early Holocene. The Rio Hondo fan pushed the Pecos River eastward into collapse depressions at this time as well, which allowed the Rio Hondo to build meanderbelts across the Pecos River braid plain to the south during the early to middle Holocene. Pecos River meander morphometry clearly distinguishes late Holocene to historic meanderbelts from those of the early Holocene, as meander amplitudes and wavelengths greatly increase up to a maximum 2 km wide meander with a wavelength of 1.6 km, recorded from a 1940-vintage aerial photograph. Extensive man-made meander cutoffs were constructed in the 1950s and 1960s, resulting in a modern channel that exhibits few, small meanders that are typically locked into bedrock or former braided channel courses.