Early Pliocene paleovalley incision during early Rio Grande evolution in southern New Mexico
— D. J. Koning, A. P. Jochems, and M.T. Heizler
Stratigraphic relations in the Truth or Consequences area indicate that the early Rio Grande incised and backfilled twice over 0.6 my, forming two 20-m-deep paleovalleys, shortly after this paleoriver propagated into southern New Mexico. Paleovalley incision is recognized by buttress unconformities between different-aged, lithologically distinctive axial and piedmont deposits. Older (pre-paleovalley) deposits correlate to the Palomas Formation basal transitional unit (map unit Tplt), which includes western-derived piedmont deposits and some of the earliest Rio Grande deposits in the Palomas and Engle Basins. The first (older) paleovalley was backfilled with axial-fluvial sediment consisting of a coarse-grained, basal conglomerate overlain by sandstone and pebbly sandstone; all gravel was locally derived from highlands surrounding the Engle Basin. The second paleovalley fill is distinctive because it contains extrabasinal, coarser (cobble-rich) conglomerate. To the west, the top of the Tplt unit is overlain by coarser-grained and lesser cemented piedmont deposits of the Palomas Formation. These volcaniclastic deposits interfinger eastward with axial-fluvial deposits overlying both aforementioned paleovalleys; these younger axial-fluvial deposits are composed of sand with 5–20% pebbly beds that lack extrabasinal gravel. Radiometric dating of basalt clasts and cryptomelane provide age control for paleovalley formation. A basalt clast dated at 5.06±0.02 Ma (40Ar/39Ar age), collected ~4 m below the top of Tplt, provides a maximum age for both the top of this unit and incision of the older paleovalley. Cryptomelane precipitated at the top of the older paleovalley fill is 4.87±0.05 (previously published 40Ar/39Ar age). Thus, incision and back-filling of the first paleovalley happened between 5.1 and 4.87 Ma , and incision of the second, nested paleovalley occurred after 4.87 Ma. A basalt cobble collected 6 m above the base of the coarse-grained, lesser-cemented piedmont deposits returned an 40Ar/39Ar age of 4.49±0.03 Ma. This stratigraphic horizon projects to 10–12 m above the base of the younger axial-fluvial unit that overlies the second paleovalley; the horizon also underlies (by at least 15 m) the ~3.6–3.3 Ma Repenning fossil site. Thus, aggradation of the second paleovalley continued to ca. 4.5 Ma. The relatively quick succession of two ~20-m-deep incision/backfilling events within 0.6 my, each incising to a similar base level, suggests a paleoclimatic driver that caused notable fluctuations in sediment-water discharge ratios similar to later Pleistocene events. We propose that a similar buried paleovalley may exist between the Palomas and Mesilla Basins, which might explain the general lack of pre-3.6 Ma axial-fluvial deposits in that region.
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- Koning, D. J.; Jochems, A. P.; Heizler, M.T., 2018, Early Pliocene paleovalley incision during early Rio Grande evolution in southern New Mexico, in: Las Cruces Country III, Mack, Greg H.; Hampton, Brian A.; Ramos, Frank C.; Witcher, James C.; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 69th Field Conference, pp. 93-108.