73-2 Volcaniclastic Depositional Processes during Talkeetna Arc Infancy as Recorded by the Upper Triassic Shuyak Formation, Kodiak Archipelago, Alaska

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes (Posters)

Poster Booth No.: 272

Presenting Author:

Brandon Keough

Authors:

Keough, Brandon¹, Johnson, Lauren², Reifeis, Logan³, Ridgway, Kenneth⁴, Eddy, Michael⁵

(1) Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA, (2) Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA, (3) Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA, (4) Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA, (5) Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA,

Abstract:

The Late Triassic-Jurassic Talkeetna Arc of southern Alaska is considered a classic oceanic arc crustal section. In the Chugach and Talkeetna Mountains, the Jurassic Talkeetna Formation archives volcanic and volcaniclastic depositional processes within the mature arc. Our study investigates depositional processes associated with volcanic and volcaniclastic strata deposited during and shortly after the transition from subduction initiation to early arc volcanism. These processes are recorded by the Upper Triassic Shuyak Formation, which offers a rare opportunity to examine depositional processes associated with the onset of arc volcanism in an oceanic setting.

Our mapping and stratigraphic analysis documents a gradational transition from pillow basalts interpreted to represent subduction initiation volcanism (Lower Shuyak Formation) to a 2-km-thick section of volcaniclastic strata (Upper Shuyak Formation). Detailed measured sections describe three mappable members of the Upper Shuyak Formation. The lower member consists of mudstone and thin-bedded volcaniclastic turbidites sharply interbedded with massive conglomeratic intervals with flattened tuffaceous and pumice clasts and a mafic-rich sandy matrix. The abrupt stacking of these distinct lithofacies is interpreted to represent intercalated deposits derived from different discrete edifices in the early stages of arc volcanism. The middle member is characterized by well-developed volcaniclastic turbidites interbedded with laminated mudstone and minor basalt flows. Sandstone compositions are notably more quartz-rich than in the lower member. Volcanic chert forms a syn-depositional silicic cement within most sandstone lithofacies, and soft-sediment deformation is common in mudstone-dominated intervals. We interpret the middle member to represent medial-distal deposition in an intra-arc submarine fan system. The upper member consists of ~10-cm-thick silicic tuffs interbedded with finely laminated, reworked tuff and volcaniclastic siltstone. Volcanic bombs are common. We interpret the upper member as distal submarine fan deposits containing airfall ash related to eruptions that produced the volcanic bombs. Compositional trends throughout the section show that the system became progressively more silicic beginning in the middle member. The most proximal deposits occur in the lower member, suggesting that deposition of the Upper Shuyak Formation began close to the locus of early arc volcanism and that the volcanic axis shifted away with time.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5566

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