Abstract

During late Paleozoic time in the eastern Alaska Range, a variety of depositional environments developed in response to the growth of a marine volcanic chain that was part of an arc-trench system. Sedimentation in environments within the chain was controlled largely by volcanism, tectonism, a high rate of sedimentation, and steep depositional slopes. The combination of these produced interbedded volcanic and sedimentary deposits and facies patterns by which the arc-related environments and the arc system itself can be recognized.

The existence of the chain is indicated by an extensive, apparently linear, deposit of volcanic rocks of calc-alkaline composition. More than half of the volcanic rocks are pyroclastics indicating the predominately explosive behavior characteristic of arc-related volcanism. The presence of thick, coarse-grained pyroclastics and numerous hypabyssal intrusives with compositions nearly identical to the volcanic rocks is evidence that the depositional environments were close to active vents and probably were located along the axis of the volcanic chain. Furthermore, steep depositional slopes, typical of the axial region of modern chains, are evident from the abundance of slump and slide structures in nearly all sedimentary sections. Sedimentary deposits throughout the arc are mostly marine and consist of interbedded graded sandstone and laminated mudstone indicative of gravity-induced sedimentation below wave base. An analysis of volcanic and sedimentary facies patterns suggests that most of the volcanic sedimentary materials accumulated in large submarine aprons each of which developed around a nearby vent or group of vents.

The distribution of late Paleozoic rocks beyond the area studied is too poorly known to establish for certain the trend and polarity of the arc-trench system. However, possible trench, accretionary prism, frontal arc, or marginal basin deposits consisting of ophiolites (?), black shales, cherts, and basic volcanics of either known or possible late Paleozoic age occur in the central Alaska Range and parts of southern Alaska. These rocks are distributed in a pattern, which when combined with the distribution of the volcanic chain deposits, suggest that the late Paleozoic arc-trench system trended northwest obliquely into the Denali Fault.