New Mexico Geological Society Annual Spring Meeting — Abstracts

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Faulting plays an important role in shaping the landscape and influencing the distribution, preservation, and post-depositional modification of syntectonic strata in the Albuquerque Basin, New Mexico. The Sand Hill and San Ysidro fault zones influenced sedimentation at two different scales. At the fault-block scale, normal faulting controlled the thickness, distribution, and preservation of synrift Santa Fe Group strata. At smaller scales, fault activity generated relatively narrow fault-proximal sedimentary wedges that record localized accommodation above the early Pleistocene Llano de Albuquerque geomorphic surface. These two scales of fault–stratigraphic interaction provide insights into Pleistocene landscape evolution and highlight challenges of reconstructing paleoseismic records in extensional basins experiencing low rates of deformation.

Along the Ceja del Rio Puerco, regional fault–stratigraphic relationships show strong structural control on the Plio–Pleistocene Ceja Formation. On the footwall of the Sand Hill fault zone, Ceja strata locally incised into Miocene deposits of the Arroyo Ojito and Cerro Conejo Formations, whereas thicker and more complete sections of Santa Fe Group strata are preserved on adjacent hanging walls of the San Ysidro fault zone. Along the proximal hanging wall of the Calabacillas segment of the San Ysidro fault zone, a fault-controlled wedge overlies the Ceja Formation and pinches out basinward within ~3 km of the fault trace.

The most continuously exposed of these fault-controlled wedges occurs along the Calabacillas segment, where a 26-m-thick hanging-wall wedge is exposed for nearly 1 km along fault strike. Detailed mapping and outcrop analysis document at least 18 laterally continuous, paleosol-bounded subunits formed between 1.8–1.3 Ma and before ~130 ka. This section consists predominantly eolian and sheetwash deposits with minor colluvium, all of which thicken toward the formative fault. These strata are interpreted to record repeated alternations between short-lived depositional pulses and prolonged intervals of geomorphic stability. Stacked, laterally continuous paleosols and low long-term net accumulation rates (~14–20 m/My) indicate that deposition was strongly modulated by climatically driven variations in sediment supply and pedogenesis. Low long-term slip rates (10–100 m/My) and long recurrence intervals (10–100 ky) overlap major modes of Pleistocene climatic variability. As a result, climatically driven variations in deposition and soil development may obscure rupture-related stratigraphic signals. The correspondence between paleosol stacking patterns and paleoclimate proxies further supports a strong climatic influence on wedge architecture. Improved geochronologic control coupled with a more comprehensive analysis of fault-controlled sedimentation can be used to improve rupture histories for low-slip-rate faults in semi-arid, eolian-prone basins.