SHEET-LIKE FLUVIAL SAND BODIES AND INCISED VALLEYS IN THE FLUVIO-LACUSTRINE TRIASSIC DOCKUM GROUP, PALO DURO CANYON, TEXAS PANHANDLE, USA

Tara L. Benda¹ and Lee F. Krystinik^2
1 Stratigraphic Prediction and Analysis, Upstream Technology, ConocoPhillips, Houston, TX, [email protected]
² Krystinik Litho-Logic, Fort Worth, TX, [email protected]

Fluvio-lacustrine deposits of the Triassic Dockum Group are the product of a complex interplay of tectonics, climatically controlled lake-level fluctuation, and variable sediment point sources. Several scales of incised channel forms and linear sediment conduits occur within the Dockum Group.

Faults control the major sediment conduits into the basin and all sand bodies discussed occur within these conduits. Interfluves are nearly non-depositional areas characterized by a range of arid paleosol features (caliche, rhizocretions and incipient paleosols) on lacustrine mudstone. At the basin margins, fluvial channels may intercalate with flood-plain mudstone or incise into lacustrine mudstone. In more basinal areas, fluvial mudstone may be rare or absent and coarse sediment is most commonly in erosive contact with subjacent lacustrine mudstone.

Elongate, linear sediment conduits contain a number of fill types. A typical highstand stratal succession consists of reddish lacustrine mudstone gradationally overlain by cleaning upward, muddy deltaic deposits. These fine-grained deltaic deposits are erosively overlain by coarse-grained, sharp-based sandstone and conglomerate deposited within downlapping clinoform sets of falling-stage deltaic deposits. Evidence of multiple, high-frequency lake-level fluctuations occurs within these falling-stage deltaic lobes. The last active highstand or falling-stage deltaic distributaries appear to be the precursors to incised valleys cut during rapid falls of lake level. These sharply incised valleys and arroyo-like channels cut across sediment bypass areas between highstand and lowstand depocenters.

Sheet-like fluvial sandstone and conglomerate bodies appear to form during times of low accommodation and moderate sediment supply, during stillstands of lake level. Low to moderate sediment supply allows repeated, fairly deep incision along unconformities, yet sediment supply is high enough to cause frequent avulsion and re-incision. Fluvial sheets are a product of amalgamated unconformities and large amounts of the sedimentary record are interpreted to have been removed during successive fluvial incisions and lacustrine flooding events.

Log shapes are made complex by the range of sharp-based successions and the presence of older, unrelated deltas immediately below incisive units. Because distributaries, falling-stage deltas and incised valleys are all sharp or erosively based successions, sub-surface mapping using well log data can be problematic, yet these depositional systems have radically different reservoir geometries and interconnectivity. Outcrop work, cores and image logs can provide critical information for reservoir prediction.

Understanding the intimate interaction of sediment supply, lake level and tectonics aids in understanding and predicting basin-fill architecture, key sediment conduits and the reservoir character of the fill of these conduits.