New Mexico Geological Society Annual Spring Meeting — Abstracts

Little is known about fault controls on fluid flow through unconsolidated sediments, although many important aquifers in the United States are found in such basins. The Sand Hill fault (SHF) is a major north-south trending, high-angle normal fault associated with the Rio Grande Rift. This growth fault locally separates the syn-rift deposits of the upper and lower Santa Fe Group. Our project is part of a collaborative effort trying to characterize fluid flow through unconsolidated sediments in faulted basins, for the purpose of more accurate fluid flow modeling of these systems. The focus of our field is study the hydrogeology of the hanging wall sediments associated with the SHF in the Pliocene to Pleistocene upper Santa Fe Group.

Physical characteristics of sediments, such as grain size and sorting, act as indicators of possible pathways and barriers to fluid flow. Calcic soil horizons, which are common in the Albuquerque Basin, can change the hydrologic properties of a sedimentary unit by depositing calcium carbonate around and between grains, closing off pathways for fluid flow. Our research utilizes 1 :8,000 lithofacies mapping to document lateral changes and several measured sections to document vertical/temporal changes. The measured sections document lithofacies changes in greater detail than the mapping, allowing a better idea of lithologic variations within individual mappable units, which may influence permeability. Calcic soils are documented as a map overlay and incorporated into the measured sections.

The lithologic units are divided into five mappable units within two major depositional environments. In the upper Santa Fe Group we document basin scale fluvial sediments present at the time of fault propagation and fault-related syn-tectonic depositional wedge (STDW) deposits that occur as a result of unstable, fault-related, surface morphology due to fault propagation. The upper Santa Fe Group fluvial deposits are divided into three units: gravel-and conglomerate-dominated, sand-dominated, and silt-and mud-dominated units. The STDW consists of a poorly-sorted colluvial wedge unit, an eolian unit and a well-sorted, fault-parallel fluvial unit that contains reworked colluvial wedge and upper Santa Fe Group fluvial sediments. For mapping purposes the eolian and fault-parallel fluvial sediments within the STDW are grouped together as both represent well-sorted sands.

The STDW sediments are commonly associated with soil-bounded stratigraphic intervals, within the upper Santa Fe Group, representing episodic movement along the SHF. The fluvial facies acts as background sedimentation within the basin. The calcic soils represent periods of relative tectonic and landscape stability between rupture events along the SHF. To test the viability of these lithologic units as hydrostratigraphic units, permeability measurements are being undertaken using a variety of methods: air minipermeameter, grain-size analysis, and falling-head permeability.