Tuesday, March 20, 2018 - GSA Luncheon

 TOPIC: Sequence-stratigraphic framework of the Middle Jurassic Chinitna Formation, Cook Inlet forearc basin, south-central Alaska

Noon Luncheon 11:30-1:00 pm

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The Chinitna Formation is the latest Middle Jurassic forearc basin record of the Talkeetna oceanic island arc. Recent work along the arc-proximal Iniskin Bay–Tuxedni Bay outcrop trend on the northwest side of lower Cook Inlet provides new insights into the Chinitna, which comprises the Tonnie Siltstone (Bathonian–Callovian) and Paveloff Siltstone (Callovian) Members. Outcrop-based observations, process–response sedimentology, 1:63,360-scale geologic mapping, and architectural analysis of mountain-scale exposures are the foundation of a sequence-stratigraphic interpretation for the Chinitna, which lies between the Middle Jurassic Tuxedni Group and Upper Jurassic Naknek Formation.

The Chinitna in outcrop is commonly ca. 700 m thick, with each member representing approximately half of this total thickness. Successions of bioturbated siltstone with subordinate, sharp-based, graded sandstone are common in the middle and upper parts of each member. Abundant fossils (e.g., ammonites, pelecypods, belemnites), discrete trace fossils (e.g., Thalassinoides, Planolites, Phycosiphon), and woody debris are present in these lithostratigraphically characteristic exposures of the Chinitna. These observations suggest that mud-prone sedimentation was intermittently punctuated by higher energy sediment gravity flow events that transported sand to shelfal environments, including prodelta settings. However, thick, coarse-grained basal successions in each member impart prominent member-scale stratigraphic cyclicity to the Chinitna. These basal units principally comprise tabular-bedded and channelized sandstone and conglomerate that sharply overlie surfaces varying from planar to exhibiting several tens of meters of erosional relief. We also observe convolute stratification and very thick, texturally structureless beds and infer that the basal successions record deposition in high energy deltaic and associated shoreline-proximal settings.

The Tonnie and Paveloff are each interpreted as third-order (i.e., 106 years duration) stratigraphic sequences. The coarse-grained basal units are regressive deposits of lowstand systems tracts (LSTs) that mark base-level fall and early rise associated with onset of each cycle. Overlying, fine-grained, fining-upward successions in each member reflect waning deltaic influences as near-shore environments were transgressed by landward/northwestward-shifting shorelines of transgressive systems tracts (TSTs) during continued base-level rise, transitioning into mud-prone shelfal settings. TST deposits are overlain by coarsening- and thickening-upward successions in the upper parts of each member, recording normal regressions of highstand systems tracts (HSTs) as probable clinoforms of delta- to slope-scale relief prograded basinward/southeastward during later stages of base-level rise. Each Chinitna member HST was terminated by base-level fall associated with establishment of an overlying sequence boundary.

This sequence-stratigraphic interpretation delineates timing for transport of coarse, LST detritus into the basin, which bears on the potential distribution of hydrocarbon reservoir facies in the underexplored Jurassic stratigraphy. We also discovered oil-stained outcrops in the Chinitna—one in each of the two LSTs—neither of which are associated with outcrop-scale fractures, suggesting that the migrated oils occur in intergranular porosity. Furthermore, the size and context of the Tonnie Siltstone Member LST oil-stained locality provides a potential outcrop analogue for a stratigraphically trapped conventional hy¬drocarbon accumulation of oil field scale. These outcrop-based insights are positive indications for continued exploration in Cook Inlet, further challenging the paradigm that Mesozoic units have low potential as conventional oil and gas reservoirs.

Oblique aerial view southeastward of the Triangle Peak area south of Tuxedni Bay, northwestern region of lower Cook Inlet. Both members of the Chinitna Formation, the Tonnie Siltstone (Jct) and Paveloff Siltstone (Jcp), crop out in this excellent exposure. Stratigraphic architectural units (subscripted labels) are the building blocks of our sequence-stratigraphic interpretation for the Chinitna Formation (see inset). The underlying Bowser Formation (Jtb) (Tuxedni Group) and overlying Naknek Formation (Jn) are visible at lower right and upper left of photograph, respectively. Dashed and dotted black lines are member and formation contacts (long dash), architectural unit contacts (short dash), and architectural subunit contacts (dotted); dashed orange lines highlight onlap surfaces; red arrows mark stratal terminations; thick black lines with slip arrows are faults; thin yellow lines delineate steep ridges that disrupt perspective; Mbr.—Member. Sequence-stratigraphic surfaces are explained in the inset key. See labeled stratigraphic thicknesses for sense of scale. Photograph by T.M. Herriott.


Trystan Herriot, Alaska Division of Geological & Geophysical Surveys, Fairbanks, AK

Trystan M. Herriott received B.S. and M.S. degrees in geological sciences from the University of California at Santa Barbara. During his graduate years at Santa Barbara, he examined ancient and modern volcanic arc settings in the Chilean Andes and Pacific Northwest. Trystan’s studies along the Backbone of the Americas continued northward when he joined the Alaska Division of Geological & Geophysical Surveys in 2009. In his work with DGGS, Trystan employs principles of basin analysis to unravel geologic histories in northern, interior, and southern Alaska. Trystan remains optimistic that future discoveries of Jurassic dinosaur fossils in south-central Alaska will garner broader interest in the region’s earliest arc–forearc record.

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