Projects - Gallup Sandstone
Project 2: Facies Architecture of the shoreface to delta front transition, Gallup Sandstone, New Mexico
The Cretaceous Gallup sandstone of New Mexico has been interpreted as wave-dominated shorefaces associated with a barrier-island depositional setting. Scouting of these outcrops in the summer of 2004, around Rock Ridge, south of Ship Rock, shows a superb examples of the transition between a fluvial, flood-dominated delta front laterally into a wave-dominated shoreface. Earlier work did not recognize the fluvial-nature of the parasequences along Rock Ridge (Fig. 6). Our scouting showed poorly-bioturbated mudstones interbedded with sharp-based, normally graded sandstone beds displaying Bouma sequences (Fig. 7). Not exactly the features typically associated with wave-dominated shorefaces.
Figure 6. Heterolithic parasequences, interpreted as fluvial-dominated, in the Gallup sandstone at Rock Ridge, New Mexico. These were previously interpreted as wave-dominated shorefaces.
These heterolithic parasequences pass laterally into homogenous and pervasively bioturbated to cross bedded sandstones interpreted as "classic" shorefaces (Fig. 8). These lateral facies relationships suggest a mixed river- and wave-influenced delta. We have recently developed a model for wave-influenced asymmetric deltas that predict specific relationships that link shorefaces and delta fronts (Bhattacharya and Giosan, 2003). Essentially, the fluvial-dominated delta fronts are predicted to lie downdrift of the sandier, more homogenous and potentially better reservoir quality shoreface systems in the updrift region.
Figure 7. Unbioturbated heterolithic strata (left) are associated with normally-graded sandstone beds displaying Bouma sequences (right) and interpreted as possible river-produced, hyperpycnal turbidites. These observation are not compatible with a wave-dominated shoreface interpretation and suggest far more fluvial influence.
Figure 8. Thick sandy parasequence, about 1 mile east of the outcrops shown above show features more typical of a wave-dominated shoreface.
Our focus in the project will be to map the bedding relationships between the shoreface and delta front. Paleocurrent information and ichnological analysis will be critical to determine the detailed paleogeography and the orientation of the river plume versus the longshore drift system.
Two MS students will initiate field work on this unit this summer. Additional information on both proposed projects can be found by contacting Professor Bhattacharya as listed above.
