New collections of floral and faunal remains were recovered from late Paleozoic sediments of the Mount Dall conglomerate in the Alaska Range of south-central Alaska. This isolated unit's type section is ∼1500 m thick and comprises thick to very thick conglomerate beds with interbedded sandstones and siltstones in a series of fining-upward intervals each tens of meters thick. The unit is interpreted to be a coastal braidplain deposit of Early Permian age in the upper Farewell terrane (Mystic subterrane sequence). Genus-level taxonomic composition of paleobotanical collections from lenticular mudstones to siltstones is discussed with regard to taphonomy and the interpreted lowland paleoenvironment of deposition. Poorly to moderately preserved megafossil compressions and impressions of the foliage genera Pecopteris , Zamiopteris , Rufloria , Angaropteridium , Cyclopteris , and Cordaites are consistent through several hundred meters of section and suggest a locally dense floral community. Horizons with sideritic rhizoliths indicate the presence of immature soils. The co-occurrence of these foliar and reproductive organs in the Mount Dall conglomerate suggests a mixed phytogeographic affinity to both the temperate Angaran Floristic Province of northern Pangea and the Euramerican Province of lower paleolatitudes. The brachiopod genera ? Stenoscisma and ? Schuchertella also were recovered and indicate a coastal depositional setting. These new biogeographic data complement exclusively marine zoogeographic data from the Farewell terrane's older strata and may be used to test hypotheses regarding the paleogeography of this displaced continental fragment. The paleofloral data support the placement of this terrane within a midlatitude climate belt during the Early Permian.

The Kahiltna assemblage of southern Alaska crops out in an 800-km-long belt that forms the core of much of the rugged Alaska Range. New sedimentologic, provenance, and geologic mapping data suggest that the Kahiltna assemblage exposed in the western Alaska Range represents a late Early Cretaceous to Late Cretaceous marine basin that formed in response to oblique collision between a composite island-arc terrane and the Mesozoic continental margin of North America. The Kahiltna assemblage in the study area crops out in two belts located north and south of the Denali fault system. Measured stratigraphic sections show that the Kahiltna assemblage in the southern outcrop belt has a minimum thickness of 5560 m and consists of eight siliciclastic lithofacies that represent tabular and weakly channelized mixed sand-mud submarine-fan systems that developed in a base-of-slope environment of deposition. Our analysis of the Kahiltna assemblage located north of the Denali fault indicates the presence of similar lithofacies along with additional strata that we interpret to represent outer-shelf and/or upper-slope (slope apron) depositional environments. Geologic mapping for this study identified the depositional basement of both outcrop belts as Upper Triassic to Lower Jurassic marine-volcanic and volcaniclastic strata that form the upper part of the Mystic subterrane. Detrital zircon data constrain the depositional age of most of the Kahiltna assemblage in the study area to Early Cretaceous time (Aptian or Albian) or later and suggest a significantly younger timing of basin development than previously recognized. Compositional data indicate that sandstone and conglomerate of the Kahiltna assemblage were derived from both Mesozoic continental margin and composite island-arc terrane sources. Modal sandstone compositions (n = 41) are consistent with a mixed arc and recycled orogen provenance (Q 23 F 9 L 68 ; Qm 11 F 9 Lt 80 ). Detrital zircons from sandstone collected in the lower part of the Kahiltna assemblage yield Precambrian (32%), Paleozoic (12%), and Mesozoic (56%) U-Pb ages. Concordant ages are consistent with the age distributions of Proterozoic, Devonian, Mississippian, and Triassic-Jurassic plutonic rocks of the former continental margin that formed the northern boundary of the Kahiltna basin. Plutons of the Talkeetna and Chisana arcs, part of the composite island-arc terrane located south of the basin, also probably contributed to the abundance of detrital zircons with ages between 200 and 163 Ma and between 124 and 106 Ma, respectively. Our new findings indicate that by Early Cretaceous time, the North American continental margin and composite island-arc terrane were in close enough proximity for both to contribute sediment to the Kahiltna basin. Stratigraphic, structural, and geochronologic relationships presented here, combined with previous regional studies of Mesozoic strata in the suture zone, suggest that the Kahiltna assemblage is the product of oblique island-arc terrane collision. Oblique collision resulted in the juxtaposition of continental margin and oceanic strata within thrust sheets along the closing suture zone. Dominantly west- and southwest-directed submarine-fan systems transported detritus axially away from the closing suture zone and into the along-strike marine basin represented by most of the Kahiltna assemblage exposed in the western Alaska Range. Comparisons with along-strike uplifted Mesozoic marine basins suggest westward time-transgressive closure of a suture zone that extends from British Columbia to southwestern Alaska.