| Abstract |
- 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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