Outcrop well logs reveal stratigraphic and glacioeustatic records in the Fusselman Formation and Montoya Group, west Texas and New Mexico, U.S.A.

ABSTRACT

Despite the importance of equivalent strata in multiple basins as oil, gas, and saltwater disposal reservoirs, and a recent rise in associated seismicity hazards, there remains a lack of modern outcrop studies of the Silurian to Lower Devonian Fusselman Formation (shallow-marine ramp carbonates) in west Texas and New Mexico, U.S.A. Furthermore, dolomitization and silicification, apparent lithologic homogeneity, and severe post depositional erosion prevent identification of Silurian eustatic signals that are evident in other regions. The Fusselman Formation lacks a high-resolution stratigraphic framework, hindering predictive mapping and efforts to relate this unit to coeval strata elsewhere. To address these deficiencies, this study applies a suite of high-resolution outcrop measurements, including spectral gamma ray, Leeb rebound hammer, outcrop image analysis (prevalence of vugs and chert), and reflected-light microscopy (dolomite crystal size), alongside analysis of carbonate-rock facies to six measured sections. These techniques reveal previously obscure stratigraphic relationships. The sections cover both the Fusselman Formation and the underlying Upper Ordovician Montoya Group, which was deposited in a similar setting to the Fusselman Formation but already has an established high-resolution sequence stratigraphic framework. Locations record a loosely distal-to-proximal transect.

The Fusselman Formation contains six third-order depositional sequences near its type location in the northern Franklin Mountains. Sequences FT1–FT5 (four complete, one partial) correspond to the four globally established sea-level fluctuations during the Llandovery Epoch of the Silurian. In contrast, FT6 spans the Middle Silurian (Wenlockian) through earliest Devonian but relationships to specific highstand–lowstand episodes are unclear. Post depositional karsting and burial diagenesis may mask higher-frequency cyclicity in FT6. Parts of these sequences thin, become amalgamated, or are truncated by erosion in more proximal locations. The ramp geometries and associated ramp-crest facies change in successive sequences. Shallowing-upwards successions are developed in the lower part of the Fusselman Formation (Sequences FT1–FT3) but become less distinct in the upper sequences (FT4–FT6). This study establishes the first high-resolution chronology and calibration to the global sea-level curve for the Silurian of central-southern Laurentia and demonstrates a repeatable method for identifying internal stratigraphy in homogeneous, highly altered Paleozoic carbonate successions in outcrop and subsurface.