Stratigraphy, structure, and Early Paleozoic Gastropoda of the Callahan Area, Klamath Mountains, California Public Deposited

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  • The sedimentary, metamorphic, and igneous rocks of the Callahan, California area formed on, or adjacent to, a Lower Paleozoic island arc complex which has since been tectonically disrupted. Sandstone, shale, lithic wacke, chert, banded quartzite, siliceous mudstone, conglomerate, and limestone of the eastern Klamath belt were deposited from the Middle Ordovician through the Early Devonian on top of the peridotite of the Early? Ordovician Trinity ophiolitic complex. Shallow water deposition began in the Middle Ordovician with the Facey Rock Limestone. Sandstone, siltstone, and shale of the Moffett Creek Formation probably formed as turbidity flows adjacent to the arc and were later disrupted, possibly at shallow depths in a subduction zone. Other units in the Callahan area (Callahan Chert, Gazelle Formation, quartzite of Squaw Gulch, siliceous mudstone of Thompson Gulch) may have formed in basins on the Moffett Creek Formation. The Moffett Creek Formation was derived from a primarily plutonic-metamorphic terrane, possibly a deeply eroded arc complex, while the overlying Gazelle Formation was derived from a primarily calc-alkaline volcanic source. Amphibolite and impure marbles of the Grouse Ridge Formation, which have an Early or Middle Devonian age of metamorphism, are present in the Callahan area. Jurassic-Cretaceous diorite and quartz diorite batholiths intrude the Paleozoic rocks to the west and south. The structure of the area is dominated by faults including a major steeply dipping north-south, fault separating the eastern Klamath belt from the central metamorphic belt, the Mallethead thrust fault separating upper plate phyllite and metasandstone from underlying sedimentary rocks, and a fault between at least some of the sedimentary rocks and the underlying peridotite. Facey Rock represents an isolated klippe of what once was apparently a much more extensive thrust sheet. Eighty species and forms of lower Paleozoic mollusks are described including 71 gastropods, 2 pelecypods, 2 rostroconchs, and 5 polyplacophorans. Six new genera and 7 species of gastropods are described: Biformispira isaacsoni, Boucotspira fimbriata, Ellisella greggi, Linsleyella ohnsoni, Paraliospira gradata and P. planata, and Siskiyouspira vostokovaena. In addition 13 new species are described: Maclurites klamathensis, Helicotoma griffini, H. olsoni, Mourlonia? perryi, Trochonemella mikulici, Holopea elizabethi, H. brucei, H. glindmeyeri, Gyronema liljevalli, Murchisonia (Murchisonia) callahanensis, and Coelocaulus rodneyi. Most of the specimens are silicified and are from limestone clasts in conglomerates and range in age from Middle Ordovician to Early Devonian with most specimens being Ordovician. Additional attention was given to the operculate gastropod Maclurites , a genus found in carbonate rocks worldwide. Four types of shell and 3 types of opercula were recovered in one bed indicating that the genus may be too broadly defined. Three molluscan communities are defined: Lophospira community, Murchisonia community, and Maclurites community. The communities apparently existed in shallow subtidal environments, each being slightly deeper than the preceding. This is the first extensive study to be made of the lower Paleozoic Gastropoda of western North America.
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