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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 67 (1997)No.
3. (May), Pages 469-485
Petrofacies and Provenance of a Late Cretaceous Suture Zone Thrust-Top
Basin, Cantwell Basin, Central Alaska Range
Jeffrey M. Trop, Kenneth D. Ridgway
ABSTRACT
The lower Cantwell Formation, in the Cantwell basin, provides an excellent
opportunity to study the petrofacies and provenance of siliciclastic deposits
from an ancient thrust-top basin. The Late Cretaceous Cantwell basin formed
on a series of thrust sheets along a suture zone between the allochthonous
Wrangellia composite terrane and the North American continental margin
(i.e., the Yukon-Tanana terrane). Analysis of the lower Cantwell Formation
provides an understanding of sediment transport in the interior of an ancient
suture zone. Sandstone and conglomerate compositional data indicate two
main source areas for the lower Cantwell Formation: (1) uplifted thrust
sheets, located south of the basin, associated with development of the
suture zone, and (2) a regional anticlinorium, locat d north of the basin,
along the North American continental margin. In the northwestern Cantwell
basin, petrofacies, southward paleocurrent directions, and alluvial lithofacies
record reactivation of a regional anticlinorium located in the metamorphic
Yukon-Tanana terrane. Lithic types in this part of the basin are dominated
by quartz-mica tectonite, mica schist, metamorphic chert, and other quartz-rich
metamorphic and sedimentary varieties (Q48F2L50;
Lv0Lm54Ls46). In the northeastern and
southern parts of the basin, sediment was derived from volcanic and sedimentary
strata exposed in the hanging walls of thrust faults that define the southern
margin of the basin. Source-diagnostic conglomerate clasts, spatial variations
in sandstone detrital modes, northward paleocurrent indicators, and progressively
rotated strata record syndepositional uplift of local thrust sheets along
the suture zone. The dominant sandstone grain types in these parts of the
basin are plagioclase grains and lithic fragments of argillite, radiolarian-bearing
mudstone, and metabasalt (Q44F28L28; Lv2Lm23Ls75).
Sandstone compositions documented for the Cantwell basin may be typical
of thrust-top basins that form along suture zones. The two main petrofacies
of the lower Cantwell Formation consist of a petrofacies rich in sedimentary
and volcanic detritus associated with uplifted oceanic rocks along areas
of the basin proximal to the suture zone, and a petrofacies characterized
by quartz-rich metamorphic and sedimentary detritus along areas of the
basin proximal to the deformed continental margin. Compositional data from
this study, combined with previous petrologic studies of similar basins,
show that sandstones from suture zone thrust-top basins are more feldspathic
than sandstones from "typical" foreland basins. These compositional differences
reflect a complex assemblage of accreted terran s, which include relict
volcanic arcs, batholiths, oceanic sedimentary deposits, and ophiolitic
melanges, that are accreted to continental margins along suture zones in
collisional-accretionary orogenic belts.
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