New Mexico Geological Society Annual Spring Meeting — Abstracts

Thompson (1942, NMBMMR Bulletin 17) named the Bruton Formation for a 35-m-thick section of mixed marine and nonmarine strata between Virgilian limestones below and Wolfcampian strata above in the northern Oscura Mountains (SE 1/4 sec. 32, T5S, R6E). Although Thompson's description met all criteria for introduction of a valid lithostratigraphic unit, Wilpolt et al. (1946, USGS OM-Map 61), in a map legend, renamed essentially the same lithostratigraphic unit Bursum with a Formation, with a stratotype only 15 km away (SE1/4 sec. 1, T6S, R4E). Although Bruton Formation had priority, most subsequent workers, especially USGS mappers, used the name Bursum Formation.

The original type sections of the Bruton and Bursum formations stratigraphically overlap but were not exactly equivalent. The type Bruton Formation of Thompson, who assigned it a late Virgilian age is overlain by a 5-m-thick limestone that contains the lowest local occurrence of the Wolfcampian fusulinid Schwagerina. Thompson did not assign this limestone to a formation (reflecting his emphasis on biostratigrapgy in defining the Bruton Formation), but Wilpolt et al. included it in the uppermost Bursum Formation. Our fieldwork at the Bruton Formation type section has identifled 28 m of mixed marine/nonmarine strata above the Schwagerina-bearing limestone and beneath the Abo Formationbase.

At Wilpolt et al.'s type section, the Bursum Formation overlies a 29-m-thick interval of siliciclastic red beds they identified as a tongue of the Abo Formation, an unsupportable correlation. We therefore redefine the Bruton and Bursum stratotypes to extend from the flrst siliciclastic red beds above Madera Group limestones to the highest marine limestone below Abo Formation red beds. This defines an 85-100-m-thick mappable troit of mixed nonmarine red beds and marine limestones and shales between the Madera and Abo that should be called Bursum Formation, in accordance with longstanding usage.