Facies Architecture of an ancient distributary channel, Cretaceous Gallup Sandstone, New Mexico

This study documents the along-strike facies architectural elements of an ancient distributary channel complex in Gallup, New Mexico. Three vertical measured sections along with a high resolution photomosaic were collected to complete this analysis. Paleohydraulic parameters of flow velocity, discharge and slope were calculated using field measurements of channel elements. These measurements were tested against empirical equations to check their validity when used with sparse subsurface datasets. Two storeys of inclined heterolithic stratification (IHS) with marine to brackish ichnogenera as well as bidirectional mud draped cross beds are found in the early stage channel. This channel was approximately 60m wide, 2.5 - 4.0m deep with a calculated discharge between 52.5 – 144m³/s. This channel is downstream of the backwater limit of this siliciclastic sedimentary system and therefore was subject to tidal effects. A late stage cross-bar channel incises through the top of this channels IHS which caused intermittent abandonment of this channel indicated by two upward coarsening successions of mudstone and very fine-grained sandstone which line the channel scour. The late stage (cross-bar) channel was 51m wide, 1.7 - 2.5m deep and its calculated discharge was 30.3 - 63.8m³/s. These calculations along with facies architectural analysis allow for a plan-view reconstruction of early and late stage paleochannels. Distributary branching order for this channel was estimated by assuming the trunk channel discharge of the Gallup delta is within an order of magnitude of other Turonian fluvio-deltaic systems in the Western Interior Seaway.

The heterolithic nature of these deposits can greatly alter typical fluid migration patterns established for fluvio-deltaic environments. This could lead to inaccurate reservoir characterization models and therefore this study serves as an outcrop analog for tidally influenced distributary channels.