Early Pliocene paleovalley incision during early Rio Grande evolution in southern New Mexico

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.