New geochronologic, geochemical, sedimentologic, and compositional data from the central Wrangell volcanic belt (WVB) document basin development and volcanism linked to subduction of overthickened oceanic crust to the northern Pacific plate margin. The Frederika Formation and overlying Wrangell Lavas comprise >3 km of sedimentary and volcanic strata exposed in the Wrangell Mountains of south-central Alaska (United States). Measured stratigraphic sections and lithofacies analyses document lithofacies associations that reflect deposition in alluvial-fluvial-lacustrine environments routinely influenced by volcanic eruptions. Expansion of intrabasinal volcanic centers prompted progradation of vent-proximal volcanic aprons across basinal environments. Coal deposits, lacustrine strata, and vertical juxtaposition of basinal to proximal lithofacies indicate active basin subsidence that is attributable to heat flow associated with intrabasinal volcanic centers and extension along intrabasinal normal faults. The orientation of intrabasinal normal faults is consistent with transtensional deformation along the Totschunda-Fairweather fault system. Paleocurrents, compositional provenance, and detrital geochronologic ages link sediment accumulation to erosion of active intrabasinal volcanoes and to a lesser extent Mesozoic igneous sources. Geochemical compositions of interbedded lavas are dominantly calc-alkaline, range from basaltic andesite to rhyolite in composition, and share geochemical characteristics with Pliocene–Quaternary phases of the western WVB linked to subduction-related magmatism. The U/Pb ages of tuffs and 40Ar/39Ar ages of lavas indicate that basin development and volcanism commenced by 12.5–11.0 Ma and persisted until at least ca. 5.3 Ma. Eastern sections yield older ages (12.5–9.3 Ma) than western sections (9.6–8.3 Ma). Samples from two western sections yield even younger ages of 5.3 Ma.
Research Article| May 01, 1997 Thrust-top basin formation along a suture zone, Cantwell basin, Alaska Range: Implications for development of the Denali fault system Kenneth D. Ridgway; Kenneth D. Ridgway 1Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-1397 Search for other works by this author on: GSW Google Scholar Jeffrey M. Trop; Jeffrey M. Trop 1Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-1397 Search for other works by this author on: GSW Google Scholar Arthur R. Sweet Arthur R. Sweet 2Geological Survey of Canada, Calgary, Alberta T2L 2A7, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Kenneth D. Ridgway 1Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-1397 Jeffrey M. Trop 1Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-1397 Arthur R. Sweet 2Geological Survey of Canada, Calgary, Alberta T2L 2A7, Canada Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1997) 109 (5): 505–523. https://doi.org/10.1130/0016-7606(1997)109<0505:TTBFAA>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kenneth D. Ridgway, Jeffrey M. Trop, Arthur R. Sweet; Thrust-top basin formation along a suture zone, Cantwell basin, Alaska Range: Implications for development of the Denali fault system. GSA Bulletin 1997;; 109 (5): 505–523. doi: https://doi.org/10.1130/0016-7606(1997)109<0505:TTBFAA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Cantwell Formation consists of a lower sedimentary sequence as much as 4000 m thick and an upper volcanic sequence with a maximum thickness of 3750 m that was deposited in the Cantwell basin, south-central Alaska. Previous to this study, the Cantwell basin was interpreted as a Paleogene, nonmarine (mainly fluvial), pull-apart basin that formed in response to dextral, strike-slip displacement on the Denali fault system. This study proposes that the Cantwell basin formed as part of the Mesozoic accretionary phase of deformation, prior to the development of the Cenozoic postaccretionary Denali fault system. Our reinterpretation is based on several new lines of data.(1) Age. New data based on palynologic analyses of 135 fine-grained samples indicate that the lower Cantwell Formation was deposited during the late Campanian and early Maastrichtian. On the basis of previous regional tectonic studies and this new age constraint, the formation of the Cantwell basin was coeval with regional Late Cretaceous shortening associated with accretionary tectonics in southern Alaska.(2) Depositional systems. Our analysis of the Cantwell Formation demonstrates that sedimentation occurred mainly in stream-dominated alluvial fan, axial braided stream, and lacustrine settings. These depositional systems were strongly influenced by a southward dipping, asymmetric basin floor. The presence of abundant terrestrially derived organic material, together with palynological assemblages that include marine dinoflagellates and the associated presence of oncolites, may be suggestive of a time of marginal marine influence during the deposition of the upper part of the lower Cantwell Formation. The late Campanian to early Maastrichtian timing of this possible marine influence is within the range of the Bearpaw transgressive event of the Cordilleran foreland basin and allows for regional stratigraphic correlation of the Cantwell basin with other sedimentary basins in northwestern North America.(3) Structural controls on basin formation. Mapping of intraformational angular unconformities and progressively tilted strata along the southern margin of the Cantwell basin provides direct evidence that thrust fault deformation and lower Cantwell Formation sedimentation were synchronous. Distinctive Cantwell Formation conglomerate clasts derived from the uplifted hanging walls of nearby thrust sheets adjacent to the southern basin margin also support a syndepositional thrusting interpretation. Provenance data and the concentration of proximal alluvial fan deposits along the northwestern basin margin adjacent to the Hines Creek fault indicate that it, too, was active during deposition of the Cantwell Formation.On the basis of the new data, the Cantwell basin is interpreted to have formed as a thrust-top basin (i.e., piggyback basin) along the Late Cretaceous suture zone between the accreting Wrangellia composite terrane and the North American continental margin. In contrast to previous studies, this reinterpretation of the formation of the Cantwell basin implies that the lower Cantwell Formation is not a synorogenic deposit directly associated with strike-slip displacement along the Denali fault system. Therefore, the Cantwell basin cannot be used to constrain the timing for the early development of the Denali fault system. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Eurypterids are generally rare in the fossil record, but occasionally occur in abundance. The genus Eurypterus , in particular, is well known from certain upper Silurian Lagerstatten of the northern Appalachian basin (New York and Ontario), but occurs far less frequently in the central and southern Appalachian basin (Pennsylvania, Maryland, and West Virginia, respectively). The recent discovery of an exceptionally preserved mass assemblage of Eurypterus in the upper Tonoloway Formation (upper Ludlow–Přidoli) of Pennsylvania provides new information on the behavior and life habitat of the genus in this region. Eurypterids at this locality are found in thinly laminated, calcareous shale deposited within the lower intertidal to shallow subtidal zone of a coastal mudflat or sabkha. Rare associated fauna of limited diversity, and evaporitic and desiccation features in associated beds, suggest a stressed environment with variable salinity and possible hypoxic conditions. Most eurypterids are disarticulated and fragmentary, but several fully articulated, exceptionally preserved specimens are present. Exoskeletal features and taphonomic indices values indicate a molt rather than death assemblage, and the presence of arthropod trackways suggests that Eurypterus sp. may have molted en masse in the vicinity of the burial site. Sequence stratigraphic interpretation of the site suggests that preservation of eurypterid remains is the result of occupation of ephemeral environmental (salinity/oxygen) conditions during a transgression. The occurrence of this new Lagerstatte within the upper Silurian succession of the central Appalachians, an interval which had heretofore yielded only rare, fragmentary remains, indicates that eurypterids were more prevalent in this region than previously thought.
The Chickaloon, Arkose Ridge, Wishbone, and Tsadaka Formations consist of more than 2800 m of Paleocene-Oligocene sedimentary and volcanic strata that are the products of sedimentation, volcanism, and faulting in the Matanuska Valley-Talkeetna Mountains forearc basin. These deposits provide a record of early Tertiary tectonic processes that formed the southern Alaska convergent margin. The northern margin of the forearc basin is characterized by nonmarine sandstone, conglomerate, and minor mudstone that interfinger with volcanic strata. On the basis of lithofacies, paleocurrent, and compositional data, the northern basin margin deposits are interpreted to represent southward prograding alluvial-fluvial systems. New 4 0 Ar/ 3 9 Ar ages from detrital feldspars in volcaniclastic sandstone and from igneous clasts in conglomerate suggest that these deposits were derived from Middle Jurassic and Paleocene-Eocene volcanic arc-related rocks. Stratigraphic and structural data from the northern basin margin adjacent to the Castle Mountain fault document syndepositional faulting that produced footwall growth synclines in the forearc basin. Paleocene-Oligocene strata exposed along the southern margin of the forearc basin are characterized by nonmarine sedimentary deposits that lack volcanic strata. Lithofacies, paleocurrent, and compositional data from these deposits are interpreted as recording northward prograding alluvial-fluvial systems that were derived from metavolcanic and metasedimentary source terranes of the accretionary prism. Both northern and southern basin-margin deposits merge into basin-axis deposits characterized by thick sections of carbonaceous mudstone and coal, and minor channelized sandstone. Basin-axis strata are interpreted as products of high-sinuosity fluvial and lacustrine environments that drained southwestward into the ancestral Cook Inlet basin. Unlike most previously studied ancient forearc basins, the Matanuska Valley-Talkeetna Mountains basin contains a fairly complete stratigraphic record of nonmarine sedimentation and volcanism. These deposits record multiple episodes of transpressional deformation that may be related to northward translation of the forearc basin along the continental margin, oroclinal bending of Alaska, and/or subduction of a spreading ridge. To evaluate the record of ridge subduction in Paleocene-Oligocene forearc basin deposits, two reconstructions are presented. In one reconstruction, the forearc basin and accretionary prism were translated northward as a single block with most displacement accommodated along inboard dextral strike-slip faults such as the Castle Mountain and Denali fault systems. In this reconstruction, ridge subduction beneath the forearc basin would have occurred at ca. 54-50 Ma, coeval with basinward progradation of coarse-grained deposystems, and with syndepositional displacement and growth-syncline development along the Castle Mountain fault. In the second reconstruction, in addition to displacement on inboard strike-slip faults, significant northward displacement was accommodated along the Border Ranges and Hanagita faults. These fault systems separated the forearc basin from the accretionary prism. In this reconstruction, the forearc basin and accretionary prism were translated separately and have different displacement histories; ridge subduction beneath the forearc basin would have occurred at ca. 61-58 Ma. The sedimentary record of ridge subduction in this reconstruction is represented by a basinwide unconformity and/or deposition of relatively fine-grained deposits in the forearc basin. We prefer the first reconstruction, but until additional high-resolution geochronological data are available, and the displacement histories of major fault systems are better known, both reconstructions are feasible.
Abstract The Jurassic–Cretaceous Nutzotin, Wrangell Mountains, and Wellesly basins provide an archive of subduction and collisional processes along the southern Alaska convergent margin. This study presents U-Pb ages from each of the three basins, and Hf isotope compositions of detrital zircons from the Nutzotin and Wellesly basins. U-Pb detrital zircon ages from the Upper Jurassic–Lower Cretaceous Nutzotin Mountains sequence in the Nutzotin basin have unimodal populations between 155 and 133 Ma and primarily juvenile Hf isotope compositions. Detrital zircon ages from the Wrangell Mountains basin document unimodal peak ages between 159 and 152 Ma in Upper Jurassic–Lower Cretaceous strata and multimodal peak ages between 196 and 76 Ma for Upper Cretaceous strata. Detrital zircon ages from the Wellesly basin display multimodal peak ages between 216 and 124 Ma and juvenile to evolved Hf compositions. Detrital zircon data from the Wellesly basin are inconsistent with a previous interpretation that suggested the Wellesly and Nutzotin basins are proximal-to-distal equivalents. Our results suggest that Wellesly basin strata are more akin to the Kahiltna basin, which requires that these basins may have been offset ∼380 km along the Denali fault. Our findings from the Wrangell Mountains and Nutzotin basins are consistent with previous stratigraphic interpretations that suggest the two basins formed as a connected retroarc basin system. Integration of our data with previously published data documents a strong provenance and temporal link between depocenters along the southern Alaska convergent margin. Results of our study also have implications for the ongoing discussion concerning the polarity of subduction along the Mesozoic margin of western North America.
