A SEDIMENTARY CHARACTERIZATION OF THE MT. DALL CONGLOMERATE, FAREWELL TERRANE, ALASKA AND ITS PALEOENVIRONMENTAL IMPLICATIONS

The Mt. Dall Conglomerate caps the Paleozoic strata of the southern portion of Alaska’s Farewell Terrane. A 1500-meter thick section is exposed in a synclinal structural fold within the Alaska Range, Denali National Park and Preserve, AK. This exposure in the Talkeetna Quadrangle includes Mt. Dall itself and an alpine field area of approximately four square miles. Presented here is the first detailed stratigraphic and sedimentological examination of the Mt. Dall Conglomerate including a local composite stratigraphic log, plant fossil occurrences, a local geologic map, and provenance interpretations for the varied sediment types of the massive conglomerate beds. Two distinct scales of sedimentary variability are seen within the section. The first, on a vertical scale of tens of meters, provides mappable members varying in grain-size, thickness, and clast composition. These layers are horizontally continuous within the extent of the accessible exposure and preserve clast imbrication flow directions. The second scale is on the order of meters both vertically and horizontally. Fine-grained sediments interfinger with coarser grained small channel fills containing sediments from gravel to small boulder size. These facies occur between massive conglomerate beds in horizontally variable contact. Plant macrofossils of Early Permian age are found within the heterogeneous finer grained beds. Sideritic rhizomorphs and immature paleosols are also present within sandy lenses. Evidence of a marine incursion also exists low in the section with the appearance of a brachiopod fossil. The Mt. Dall Conglomerate is made up of sediments most likely derived from lower Farewell Terrane strata of the Dillinger and Nixon Fork Sequences or related strata. Dominant and often largest in size (~5-15 cm diameter) are silty to muddy well-rounded carbonate clasts from relatively shallower water deposits with smaller (1-5 cm) red, green, black, and gray sub-rounded cherts of presumably deeper water origin being less abundant. It is hypothesized that the sedimentary character of clasts and their organization in what appears to be a braided stream depositional environment is related to the Farewell Terrane’s involvement in a collisional event and the coeval formation of a foreland basin into which these sediments were shed.