Stratigraphy, structure, and graptolites of an Ordovician and Silurian sequence in the Terra Cotta Mountains, Alaska Range, Alaska

Abstract

New Zealand, Great Britain, Idaho, and Texas. Of the 95 species described and illustrated, 2 are new species, Pleurograptus collatus and Monograptus digitatus, 2 are new subspecies, M. digitatus digitatus and M. digitatus subdigitatus, and 3 are unnamed new species. Genus Didymograptus McCoy, 1851, sensu lato Didymograptus cf. D. cognatus Harris and Thomas Occurrence.-Collections 69ACn574 and 69ACn575; Climacograptus tubuliferus Zone.

Geologic map of Alaska

“…Siltstone member is about 200 m thick and is composed of very thin-bedded, cross-laminated, yellowish-orange-weathering, silty and sandy limestone and calcareous siltstone, which contain occasional 3-m-thick pure limestone interbeds and rare argillaceous beds. Uppermost member is massive, indistinctly bedded to laminated, medium-dark-gray, finely crystalline limestone more than 100 m thick and contains minor lighter gray dolostone (Churkin and Carter, 1996). Unit is known from the McGrath quadrangle and correlative rocks are found in the Lime Hills quadrangle; the unit undoubtedly extends into the Talkeetna quadrangle, where is it included in unit D_d.…”

“…Bundtzen and others (1997a) reported late Silurian to Late Devonian conodonts from unit, based on a report in Bundtzen and others (1994) where the reported conodont has an age range of middle Silurian to Early Devonian. Churkin and Carter (1996) interpreted the lower contact of the Barren Ridge Limestone with the underlying Terra Cotta Sandstone as a thrust fault and noted that rocks of latest Silurian age are not present. On generalized map, included as part of unit D_wbl DSpf Paradise Fork Formation and correlative units (Lower Devonian and Silurian)-"Deep-water turbiditic and hemipelagic deposits of dark, thin-bedded, fissile to laminated limestone, limy shale, and siltstone.…”

“…On generalized map, included as part of unit D_wbl Stc Terra Cotta Mountains Sandstone (Silurian)-Mainly composed of sandstone and mudstone and divided into three prominent limestone members. Best described by Churkin and Carter (1996), the Terra Cotta Mountains Sandstone is distinguished from other formations in the area by abundant rhythmically interbedded sandstone and mudstone; more than three-quarters of its total thickness consists of calcareous sandstone interbedded with mudstone. In addition, the formation contains three mappable limestone members (although they are not distinguished here).…”

“…In contrast to other limestone of the Terra Cotta Mountains Sandstone, the middle limestone member contains shelly fossils at several localities. These fossils include pelecypods and straight-shelled cephalopods that are so poorly preserved that they have not been identified by genus or age (Churkin and Carter, 1996). Churkin and Carter (1996) indicate that, because of structural complications, the middle limestone member may be a repetition of the lower limestone member.…”

“…Where subdivided, includes the following units, DSbr, DSpf, Stc, and S_pl: DSbr Barren Ridge Limestone and correlative units (Devonian to Silurian? )-As defined by Churkin and Carter (1996), unit consists of "* * * two more-or-less pure limestone members separated by a siltstone member of calcareous siltstone and sandy limestone." Lower limestone is medium-dark-gray, medium-gray-weathering, well-bedded, thick-to thin-bedded, and has orange-weathering silty beds in upper half of member that form less than 50 percent of section, which is about 100 m thick.…”