A Quantitative Assessment of Lateral Variability in a Cyclic Alluvial Succession using Terrestrial LiDAR Data: Paleocene Nacimiento Formation, San Juan Basin, New Mexico
Carritt, Jeffrey; Frechette, Jedediah; Bodman, Cody; Weissmann, Gary
The early Tertiary nonmarine deposits of the San Juan Basin in northwest New Mexico are represented by the alluvial strata of the Nacimiento and San Jose Formations. The Nacimiento Formation consists of up to 500 m of fluvial deposits including variegated mudstones with intercalated paleosols and channel sand bodies. These deposits were mainly sourced from Laramide uplifts to the north (Williamson, 1996). Studies conducted on similar paleosol-rich alluvial successions have reported a three-tiered hierarchy of cyclic depositional units (Cleveland et al., 2007; Atchley et al., 2004). The smallest depositional unit these studies describe are fining-upward meter-scale successions called fluvial aggradational cycles, or FACs. Each FAC is either gradually overlain by a paleosol or the coarser base of the next succeeding cycle. These FACs also stack into ten-meter scale fining-upward FACsets that display trends in paleosol maturity. Fluvial sequences are also defined by longer scale (100 m) trends in lithology, paleosol development and FAC thickness. The three-tiers of fluvial cyclicity are attributed to episodic floodplain sedimentation, distance to channel (sedimentation source), and long-term changes in accommodation space, respectively.
In the badlands of Kutz Canyon the Nacimiento Formation is well-exposed, in places amphitheater-type exposures allow for 3-D evaluation of facies trends. Our research focuses on using spatial data collected over a selected area (~0.85 square kilometer) by a terrestrial lidar scanner to address the lateral variation of recognized FACs and FACsets. Digital photographs draped over these 3-D data produce a virtual outcrop model of the study area. The assessment of lateral variation of the cyclic deposits involves the use of numerous stratigraphic sections measured from the lidar data intensity values, and RBG data from rectified digital photographs. These digital sections are analogous to wire-line logs in form. Recognized FACs and FACsets can be statistically evaluated from these sections. Measured sections from the field appear to correspond well to the digitally produced sections. Field results also show good correspondence between the weathered and unweathered exposures, though some textural details cannot be observed through the presence of the erosion crust on outcrop.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013