Graptolite biofacies and correlation of eustatic, subsidence, and tectonic events in the Middle to Upper Ordovician of North America

• Published in: Palaios 1986-10, Vol.1 (5), p.435-461
• Main Author: Finney, Stanley C
• Format: Article
• Language: English
• Subjects: Appalachians; Arbuckle Mountains; Biofacies; Biostratigraphy; Bromides; correlation; Correlatives; Cratons; Earth sciences; Earth, ocean, space; eustasy; Exact sciences and technology; Geology; Graptolithina; Invertebrata; Kentucky; Middle Ordovician; North America; Northern Appalachians; Oklahoma; Ordovician; Ouachita Mountains; paleoecology; Paleozoic; Research Reports; Sea level; sea-level changes; Shales; Stratigraphy; structural geology; Subsidence; tectonics; Texas; transgression; United States; Upper Ordovician; Violas; West Texas
• ISSN: 0883-1351; 1938-5323
• DOI: 10.2307/3514628

Co-occurrences of key index species, discovered in Middle to Upper Ordovician strata in the Arbuckle and Ouachita mountains and in Kentucky, require revisions of the graptolite zonation. Species assemblages in the lower and upper parts of the Nemagraptus gracilis Zone are distinct enough to allow the recognition of two subzones, the Nemagraptus gracilis Subzone and the Climacograptus bicornis Subzone, respectively. The C. bicornis Subzone is largely correlative with the British Diplograptus multidens Zone and includes the classical Normanskill Shale of New York. Between the N. gracilis and Dicellograptus complanatus Zones, graptolite assemblages display marked biofacies throughout North America and support the need for a dual zonation. Critical co-occurrences show that the bases of the Orthograptus amplexicaulis Zone, as redefined herein, and the Corynoides americanus Zone are equivalent, correlate with a level low in the British Dicranograptus clingani Zone, and follow directly above the C. bicornis Subzone. Graptolite biostratigraphy indicates that the contact between the Bromide and Viola Springs formations in the Arbuckle Mountains correlates with the contact between the Womble Shale and Bigfork Chert in the Ouachita Mountains and with a hiatus between the Woods Hollow Shale and Maravillas Chert in the Marathon region of West Texas. The lithologic changes associated with these formation boundaries are interpreted to represent a eustatic drop in sea level in the Ouachita Geosyncline and a marked subsidence event in the Southern Oklahoma Aulacogen. The eustatic and subsidence events are considered to have been contemporaneous with each other and with emplacement of the Taconic Allochthon and subsidence of the Taconic Foreland Basin in the Appalachians.