CHRONICLES OF VADOSE ZONE DIAGENESIS: CONE-SHAPED IRON OXIDE CONCRETIONS, TRIASSIC TRUJILLO FORMATION, PALO DURO CANYON, TEXAS

Cone-shaped Fe-oxide concretions occur in the Late Triassic lower fluvial sandstone member of the Trujillo Formation at Palo Duro Canyon in the Texas panhandle. The dominant in situ cone orientation is apex up, body radiating down and out, with long axis perpendicular to bedding. Cubic crystal shapes at cone apices suggest pyrite (FeS₂) provided the initial iron source. Cone shapes and textures, being densely mineralized at the apex and less densely mineralized downwards, suggest downward precipitation of mineralizing fluids in the vadose zone, where partially filled pores are subject to gravitational pull. We hypothesize that the upright orientation of the concretions results from a saturated to unsaturated shift to oxygenated hydrologic conditions sometime after diagenetic precipitation of iron pyrite nodules.

Differences in cone-shaped morphologies among three lithofacies suggest that concretion shapes and sizes were affected by host rock sedimentary texture, primary bedding structure and scale, and corresponding paleohydrologic regime. Tall, slender concretions in medium- to coarse-grained sandstone likely resulted from the massive, porous nature of the beds, which permitted fluid to drain freely downwards along advection-dominated, m-scale path lengths. Tortuous, dendritic iron-staining on these concretions is interpreted to have delineated preferred paleo-flow paths in the vadose zone. Short, squat concretions in fine-grained, finely laminated sandstone are attributed to the alignment of platy clays that led to a small-scale permeability structure with a large horizontal-to-vertical permeability ratio that favored lateral fluid displacement in a diffusion-dominated zone of locally reduced permeability. Discoidal concretions in very fine-grained silty sandstone with decimeter-scale bedding planes appear to correspond to an environment intermediate between advection- and diffusion-dominated fluid flow.

The timing of diagenesis and the hydrochemical factors that resulted in the unusual shape of these Trujillo concretions, when fully understood, may provide substantive insight into events that occurred during the 200-m.y. hiatus at a major post-Triassic unconformity of the Texas panhandle.