Characterization of Length Scales and Cyclicity in alluvial Strata using Terrestrial Lidar Data: Nacimiento Formation, San Juan Basin, New Mexico

Jeff Carritt, Jed Frechette, and Gary Weissmann
University of New Mexico

Most studies that focus on the geometry and cyclicity in alluvial strata describe successions across relatively short distances which lack information on the 3D lateral variation of these deposits. In order to address this variation, our work employs terrestrial lidar and high-resolution photography to the alluvial facies of the early Paleocene Nacimiento Formation. The virtual outcrop model developed from these datasets enables the collection of quantitative measurements of paleosol and sand body length scales. These measurements provide details of subtle depositional geometries across the field site, which spans ~0.85 km2. Additionally, our investigations have shown that point cloud intensity data can serve as a lithologic proxy at this site; sections with high intensity responses are coarse-grained whereas low intensity intervals represent fine-grained beds. These point cloud data can therefore be treated as pseudo-gamma ray logs, and the response of these logs can be directly compared to known lithological variations in the field. Measured sections do correspond well to the digitally produced logs; however certain grain-size trends, for example clay to silt, are not discernible.

Wavelet analyses conducted on the point cloud intensity data revealed a cyclicity band centered at 8 meters between thin, meter-thick, sheet sandstones. These sandstone bodies are tabular in form and may be indicative of repetitive aggradational events, such as lobe switching episodes on a distributive fluvial system, though further work on a larger scale would be required to substantiate this claim.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013