New Mexico Geological Society Annual Spring Meeting — Abstracts

Although fusulinids from the Bursum Fm. are well known, virtually no information on its macroinvertebrate fauna has been reported since the formation was first recognized in 1946. The mainly nonmarine, 85-m-thick Bursum type section contains four thin marine limestone intervals bearing moderately diverse invertebrate assemblages: units 6, 21, 29, and 39, which are 7 m, 49 m, 67 m, and 80 m, respectively, above the base of the Bursum. The lowest assemblage is a hematized coquina that consists mainly of densely packed shells of the bivalves Septimyalina burmai and Myalina (Orthomyalina), with low abundances of Aviculopecten, Aviculopinna and Schizodus, the brachiopods Derbyia and Meekella, the snail Retispira aff. R. tenuilineata, and rare bryozoan and crinoid fragments. This is a nearshore, probably wave-concentrated assemblage. The unit 21 assemblage is dominated by the large (W = 75 mm) productoid brachiopod Reticulatia americana, with fewer specimens of Composita subtilita, Juresania aff. J. nebrascensis, Hystriculina aff. H. wabashensis, Meekella cf. M.. striatocostata, Derbyia, and Linoproductus. Few bivalves (Pseudomonotis hawni, Wilingia terminale, Aviculopinna) or other groups are present. The unit 29 fauna is numerically dominated about equally by brachiopods (chiefly Neochonetes transversalis, J. aff. J. nebrascensis) and by a much higher diversity of bivalves (Septimyalina, Aviculopinna, Permophorus, Parallelodon, Avtculopecten, Schizodus, Edmondia, and others). Unidentified small high-spired snails are locally abundant, often associated with small knobby algal growths; the nautiloid Mooreoceras is common, but crinoid, bryozoan and trilobite fragments are rare. The uppermost (unit 39) fossiliferous limestone is packed with small high-spired snails and Permophorus steinkerns; other bivalves (Septimyalina, Aviculopecten) and snails (Retispira, Goniasma, Naticopsis) are also present. Brachiopods, crinoid and echinoid fragments, and small pieces of wood are rare.

The macroinvertebrate faunas of the type Bursum consist mainly of taxa identical or closely related to taxa known from the Virgilian; little significant change in marine invertebrates evidently occured across the Virgilian-Wolfcampian boundary in New Mexico. Although each of the four assemblages is taxonomically distinct, they generally indicate nearshore shallow marine, perhaps locally restricted conditions. The least diverse, most mollusc-dominated assemblages (from units 6 and 39) suggest salinities that fluctuated somewhat away from normal marine values.