Sedimentology and provenance of the Upper Jurassic Naknek Formation, Talkeetna Mountains, Alaska: Bearings on the accretionary tectonic history of the Wrangellia composite terrane

Abstract

Analysis of the Upper Jurassic NaknekFormation in the Talkeetna Mountains, Alaska, documents synorogenic sedimentation in a forearc basin along the outboard (southern) margin of the allochthonous Peninsular terrane during accretion to the western North American continental margin. New geochronologic, sedimentologic, and compositional data defi ne a two-part stratigraphy for the Naknek Formation. Microfossil, megafossil, and U-Pb clast ages document early Oxfordian to early Kimmeridgian deposition of the lower 690 m of the Naknek Formation and early Kimmeridgian to early Tithonian deposition of the upper 225 m of the Naknek Formation. Lithofacies and paleocurrent data from the lower Naknek Formation demonstrate initial deposition on a high-gradient, southward-dipping basin fl oor. Submarine mass fl ows deposited poorly sorted, cobble-boulder conglomerate in proximal fan-delta environments. Gravelly mass fl ows transformed downslope into sandy turbidity currents on a muddy prodelta slope. During early Kimmeridgian to early Tithonian time, fan-delta environments were replaced by lower gradient marine shelf environments characterized by deposition of cross-stratifi ed sandstone and bioturbated mudstone. Source-diagnostic clasts, feldspathic sandstone compositions, southwarddirected paleocurrent indicators, and U-Pb zircon ages of plutonic clasts (167.6 ± 0.3 Ma; 166.5 ± 0.2 Ma, 164-159 Ma, 156.2 ± 0.4 Ma)indicate that the Naknek Formation was derived primarily from volcanic and plutonic source terranes exposed along the northern basin margin in the southern Talkeetna Mountains. Geologic mapping documents the Little Oshetna fault, a newly identifi ed northward-dipping reverse fault that bounds the northern margin of the Naknek Formation in the Talkeetna Mountains. The concentration of boulder-rich mass-fl ow deposits in the footwall of the fault in combination with geochronologic and compositional data suggest that sedimentation was coeval with Late Jurassic shortening along the fault and exhumation of plutonic source terranes exposed in the hanging wall of the fault. From a regional perspective, coarse-grained forearc sedimentation and pluton exhumation along the outboard (southern) segment of the Peninsular terrane were coeval with crustal-scale shortening and synorogenic sedimentation in retroarc basins along the inboard (northern) margin of the Wrangellia terrane (Kahiltna, Nutzotin, and Wrangell Mountains basins). We interpret the regional and synchronous nature of Late Jurassic crustal-scale deformation and synorogenic sedimentation in south-central Alaska as refl ecting either initial collision of the Wrangellia and Peninsular terranes with the former continental margin of western North America or amalgamation of the two terranes prior to collision.

Modification of Continental Forearc Basins by Flat‐Slab Subduction Processes: A Case Study from Southern Alaska

“…During Jurassic and Cretaceous time, a Jurassic intra-oceanic arc and Triassic-Jurassic oceanic plateau were accreted to the former southern Alaska continental margin (Wrangellia composite terrane on Fig. 16.2A; Nokleberg et al, 1985;Ridgway et al, 2002;Trop et al, 2005;Rioux et al, 2007). Following this episode of significant crustal growth, northward subduction of oceanic crust (present coordinates) produced a continental-margin magmatic arc, forearc basin, and accretionary prism on the recently accreted TriassicJurassic oceanic crust (Fig.…”

Thermochronology of the Talkeetna intraoceanic arc of Alaska: Ar/Ar, U‐Th/He, Sm‐Nd, and Lu‐Hf dating

“…Deformation and partial exhumation of the Talkeetna Mountains plutonic suite at this time is recorded by the Naknek Formation that unconformably overlies the Talkeetna Formation [Trop et al, 2005]. There are four K-Ar and 40 Ar/ 39 Ar biotite ages that are older than most of the hornblendes; either the younger hornblende ages were partially reset, the biotites have excess 40 [Plafker et al, 1989].…”

“…These data suggest that, at the regional scale, the Talkeetna arc in the Chugach Mountains had reached temperatures <300°C-500°C by ∼160 Ma, cooling at an average rate of >10 K/Myr; two old zircon fission track ages are compatible with this inference. This cooling may have been associated with the tectonism recorded by the Naknek Formation in the Talkeetna Mountains [Trop et al, 2005] (see below). Ar mica, and U-Th/He zircon ages from individual samples (Figure 6) reveal cooling histories in higher fidelity.…”

Intermediate to felsic middle crust in the accreted Talkeetna arc, the Alaska Peninsula and Kodiak Island, Alaska: An analogue for low‐velocity middle crust in modern arcs

“…Shuyak Formation volcanic rocks on the Kodiak archipelago contain early to middle Norian pelecypods (216.5-205.7 Ma [Connelly, 1978;Ogg, 2004b]), consistent with the older plutonic ages from this section. Finally, the Naknek Formation in the Talkeetna Mountains contains evidence for active unroofing of arc volcanic and plutonic rocks from the early Oxfordian to early Tithonian (161.2-147.2 [Ogg, 2004a;Trop et al, 2005]) and provides an absolute minimum age for the termination of arc magmatism. The range in U-Pb zircon ages from the Talkeetna plutons is equivalent to or longer than the duration of arc volcanism (Figure 9), indicating that silicic magmas were generated throughout the lifespan of the arc.…”

“…Talkeetna and Chugach Mountains zircon ages are from Rioux et al [2007]. The spans of biostratigraphic ages from the Shuyak and Naknek Formations are from Connelly [1978] and Trop et al [2005]. The time scale is after Ogg [2004aOgg [ , 2004b.…”