ABSTRACT

The lower Mineral Wells formation of the Pennsylvanian Strawn group is a siliciclastic-dominated sequence of cyclically interbedded mudstones, sandstones, and carbonates that was deposited in fluvial, deltaic, interdeltaic, and inner shelf environments during late Desmoinesian and early Missourian time. This study examined outcrops of the formation in Palo Pinto and Parker counties in north-central Texas for the purpose of describing and interpreting its facies, depositional environments, and stratigraphy.

Six depositional facies were identified in the lower Mineral Wells formation on the basis of lithology, sedimentary structures, and fossil content. The open marine facies is fossiliferous and consists of gray, beige, green, and brown mudstones, black mudstones with phosphate nodules, and limestones. The prodelta facies is largely nonfossiliferous and consists of massive mudstones, massive silty mudstones with occasional siltstone lenses, and horizontal planar- and lenticular-laminated muddy siltstones. The transgressive shoreface facies consists of cross-laminated and ripple-laminated fine-grained sandstones, highly bioturbated fine-grained sandstones, and very thinly-bedded very fine-grained sandstones. The interdistributary bay facies consists of massive mudstones with some rippled very fine-grained sandstones, highly bioturbated sandy mudstones, and muddy, very fine- to medium-grained sandstones. The swamp-marsh facies consists of gray platy mudstones containing abundant plant fragments, red mudstones containing iron concretions, and dark gray mudstones containing plant fragments and mudscracks. Lastly, the distributary channel facies consists of trough and tabular cross-laminated sandstones, wavy-bedded sandstones, and wavy-laminated and lenticular mudstones and sandstones.

The stratigraphy of the lower Mineral Wells formation is characterized by alternating progradational and transgressive sequences. Progradational sequences consist of vertical successions from open marine to prodelta to distributary channel or interdistributary bay. Transgressive sequences consist of transgressive shoreface deposits overlain by increasingly deeper marine facies.