DEPOSITIONAL ENVIRONMENTS AND SEQUENCE STRATIGRAPHY OF THE MESOPROTEROZOIC LANORIA FORMATION: IMPLICATIONS FOR TECTONIC SETTING

The Mesoproterozoic (Stenian) Lanoria Formation, located in the Franklin Mountains of westernmost Texas, is comprised of over 700 m of metamorphosed marine shelf and marginal marine strata. An investigation of physical sedimentary structures and lithologies of the Lanoria Formation has identified four primary facies associations: 1) proximal and distal marine shelf; 2) lower, middle, and upper tidal flat; 3) incised valley fill; and, 4) estuarine / deltaic. These facies associations record a dramatic northward deepening of a narrow shelf environment. Tidal influence was pronounced throughout Lanoria deposition.

Seven stratigraphic sequences, each containing multiple parasequences, have been interpreted in the Lanoria Formation. Boundaries between individual sequences are marked by basinward shifts in facies and/or regional truncation. These sequences show development of the shelf. The earliest sequences are tilted to the north and environmental shifts are limited.. Strata of the first three sequence were apparently rotated down to the north during and after deposition. These strata were then truncated by a sea-level drop and possible complete erosion at the south end of the outcrop below Sequence 4. Later sequences how much greater lateral environmental shifts and are dominantly composed of shallow-water facies.

The Lanoria was deposited on a surface of older sediments that sloped to the north. Facies and sequences show that the Lanoria filled this basin to the north and established a relatively flat shelf. The exposures do not allow an inference as to whether tectonic forcing created the pattern of sequences, however, the evidence is strong for tectonic influence on deposition and the sequence stratigraphy. The tidal influence suggests a restricted embayment setting for the Lanoria. We therefore infer an aulacogen as the tectonic setting. Additionally, the Lanoria outcrop forms part of a band of similar Mesoproterozoic siliciclastic outcrops that extend from westernmost Texas to northern Arizona.