SEQUENCE EVOLUTION AND SEQUENCE EXTINCTION: FUSULINID BIOSTRATIGRAPHY AND SPECIES-LEVEL RECOGNITION OF DEPOSITIONAL SEQUENCES, LOWER PERMIAN, GLASS MOUNTAINS, WEST TEXAS, U.S.A.

Abstract

Depositional sequences, subsequences, parasequence sets, and parasequences are well developed in strata of the North American Lower Permian Wolfcampian and Leonardian series in their type area, Glass Mountains, West Texas. Relatively long hiatuses at sequence boundaries are reflected in abrupt morphological differences in the succeeding new shallow-water carbonate fusulinacean species. The known stratigraphic ranges of many species are bounded by these hiatuses and are classic examples of how the paleontologist's recognition of derived morphologic species and the subsequent loss, or extinction, of earlier morophologic species is influenced by the hiatuses in the fossil record at sequence boundaries. The result is that ranges of morphological species commonly identify individual depositional sequences. The Neal Ranch Formation (Nealian Stage, Lower Wolfcampian) includes in its lower part seven cyclic deposits (fourth-order depositional sequences) that record significant (more than 20 m) sea-level fluctuations. In contrast, the upper part of the Neal Ranch Formation has nine cycles that represent smaller sea-level fluctuations (about 5 to 10 m). The Lenox Hills Formation (Lenoxian Stage, Upper Wolfcampian) includes three depositional sequences which are traceable westward from predominantly clastic subtidal, intertidal, supratidal, and nonmarine facies into carbonate subtidal shelfal and biohermal facies, and then into relatively deeper-water facies of interdistributary basins and conglomeratic deltas. In the western Glass Mountains, the highest part of the Lenox Hills Formation, depositional sequence LH-3, was eroded during a major post-Lenoxian and pre-Leonardian sea-level lowstand and is represented only in huge boulders and cobbles in debris beds of the overlying Decie Ranch Member at the base of the Leonardian Skinner Ranch Formation. The Hess Formation (Hessian Stage, Lower Leonardian) comprises limestone depositional sequences that are well displayed in a carbonate-platform facies in the eastern Glass Mountains, where they have many parasequences and parasequence sets. Seven Hessian depositional sequences are present, each separated by sequence-bounding unconformities and each having distinctive species of fusulinaceans. The lower four Hessian depositional sequences have well-defined parasequence sets and parasequences. Physically, the hiatuses at the top of the fourth and seventh Hessian depositional sequences suggest longer durations than others within the Hessian. Equivalent beds in the western Glass Mountains include extensive sea-level lowstand wedges of coarse boulders, cobbles, and pebbles, interbedded with calcarenitic, sandy, silty, and often radiolarian-rich turbidites. The Cathedral Mountain Formation (Cathedralian Stage, Upper Leonardian) includes at least one, and probaby two, depositional sequences. An unconformity separates the lower depositional sequence from remnants of an upper one. The mainly carbonate upper part, the lowest beds of the original Word no. 1 limestone member, is discontinuous, deeply weathered, and irregularly preserved beneath a major unconformity and major erosional surface which represent the worldwide fall in sea level referred to as the mid-Permian unconformity, or sea-level event. The lower Nealian fourth-order sequences have overlapping ranges of species of Triticites, Leptotriticites, Schwagerina, and Paraschwagerina, and the upper Nealian sequences have overlapping ranges of species of Leptotriticites, Triticites, Schwagerina, Pseudoschwagerina, Eoparafusulina, and Paraschwagerina. Within the Lenoxian, each of the three depositional sequences contains distinctive species of Pseudoschwagerina, Schwagerina, Eoparafusulina, and Paraschwagerina. Schwagerina crassitectoria characterizes the lowest Hessian depositional sequence, H-1, and distinctive depositional breaks within H-1 have subspecies-level differences. Hess depositional sequences H-2 to H-7 have a progression of increasingly advanced species of primitive Parafusulina. Parafusulina durhami and advanced morphotypes of P. leonardensis occur in Cathedralian sequence 1 and its basinal equivalents. Taxonomic revisions of several fusulinacean species include the recognition of Schwagerina glassensis n. sp., S. lenoxhillsensis n. sp., S. tewilliams n. sp., S. ironmountainensis n. sp., Pseudoschwagerina elongata n. sp., P. alacranensis n. sp., P. huecoensis n. sp., and P. huecoensis eohuecoensis n. subsp.