Depositional facies and environments of the lower Mineral Wells formation, Pennsylvanian Strawn group, north central Texas

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. Eight 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, very thinly-bedded very fine-grained sandstones, and sandy and silty mudstones. The interdistributary bay facies consists of massive clayey mudstones with some rippled very fine-grained sandstones, highly bioturbated sandy mudstones and muddy sandstones, and stratified very fine-to medium-grained sandstones. The swampmarsh facies consists of gray platy mudstones containing abundant plant fragments, red mudstones containing iron concretions, and dark gray mudstones containing plant fragments and mudcracks. The distributary channel facies consists of trough and tabular cross-laminated sandstones, wavy-bedded sandstones, and wavy-laminated and lenticular mudstones and sandstones. The meanderbeft facies consists of trough cross-bedded fine-grained sandstones, massive conglomerates, rippled very fine-to fine-grained sandstones interlayered with muddy sandstones and sandy mudstones, and horizontal planar-and cross-laminated fine-to medium-grained sandstones interlayered with cross-bedded and massive conglomerates. Lastly, the aggrading channel facies consists of large scale trough cross-bedded, wavy-bedded, and rippled medium-grained quartz sandstones and cherty pebble conglomerates. The stratigraphy of the lower Mineral Wells formation is characterized by alternating progradational and transgressive sequences. Progradational sequences typically 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.