Tuesday, November 17, 2015 AGS Luncheon

"Sequence Stratigraphic Framework of the Upper Jurassic Naknek Formation, Cook Inlet Forearc Basin, South-Central Alaska"

Noon Luncheon 11:30-1:00 pm

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The Naknek Formation records Late Jurassic forearc basin sedimentation and crops out discontinuously along an ~1000 km trend in southern Alaska. Recent sedimentologic and stratigraphic studies of the Iniskin–Tuxedni bays outcrop belt have yielded the first depositional environment interpretations for the Naknek members of lower Cook Inlet (in ascending order): Chisik Conglomerate (fan delta), lower sandstone (shelf), Snug Harbor Siltstone (outer shelf and slope), and Pomeroy Arkose (base of slope and basin floor). Furthermore, geologic mapping of the Iniskin–Tuxedni area led to the discovery of three deep-water paleo-canyons in the upper Snug Harbor–lower Pomeroy interval. These erosive-based, seismic-scale features were incised into chiefly fine-grained and tabular-bedded slope strata of Snug Harbor and filled with channelized, tabular, and lobate packages of dominantly Pomeroy sandstone. Canyon margins likely transitioned from being chiefly erosional to chiefly aggradational as the depositional system matured, as indicated by anomalously thick canyon-margin successions of Snug Harbor that suggest large-scale constructional confinement via master levees.

Recognition of these deep-water canyons and an expanded analysis of our depositional systems work permit the first sequence stratigraphic interpretation for the Naknek Formation. The basal members—Chisik Conglomerate and lower sandstone—overlie the Middle Jurassic Chinitna Formation along a sequence bounding unconformity (basal surface of forced regression (BSFR)/SB–1) and constitute a lowstand systems tract (LST–1). A transgressive surface (TS) caps LST–1 at the base of Snug Harbor (outer shelf) and marks onset of a transgre­­ssive systems tract (TST). A somewhat thicker-bedded and coarser-grained depositional motif commences in the mid-Snug Harbor (slope), defining the base of a highstand systems tract (HST) above a maximum flooding surface (MFS) at the top of TST. Erosional establishment of the deep-water canyons terminated HST at a sequence boundary (BSFR/­­­SB–2), which in inter-canyon areas is recognized as a correlative conformity. The lower Pomeroy constitutes the deep-water aspect of a lowstand systems tract (LST–2). Thus, a complete, probable third-order (i.e., 106 years duration) stratigraphic sequence occurs in the lower three members of the Naknek, with the noted exception that Snug Harbor strata locally crop out as basal canyon fill and levee deposits above BSFR/SB–2 that marks onset of a renewed accommodation succession cycle.

Key aspects of this sequence stratigraphic interpretation include: 1) Dominant sediment supply signals are recorded by LST–1 and LST–2. These lowstand systems tracts suggest episodic tectonic activity along the Bruin Bay fault system exhumed plutons of the Talkeetna arc, providing a prolific source of arkosic sediment. 2) TST and HST reflect periods of alternating balance between accommodation (favored during TST) and sediment supply (favored during HST). The relative contributions of tectonism and eustacy during this inter-lowstand systems tracts time is not known, but sediment supply may have diminished during a tectonically quiescent period in the arc–forearc region. 3) A shelf–slope–basin floor depositional profile was established in the study area by progradation of clinoforms during HST, with incipient erosional development of canyons along a relatively high gradient slope. Accumulation of bypassed, coarse-grained detritus of LST–2 occurred along deep-water reaches of diminishing gradient at and beyond the base of slope, forming a clastic wedge of Pomeroy strata that may fine and thin abruptly basinward of the outcrop belt. Field observations strongly suggest that depositional elements of the lower Pomeroy (LST–2) backstepped and onlapped the relict Snug Harbor slope (HST), an example of a stratigraphic relationship that is common to seismic data sets but rarely directly observed in outcrop.

This sequence stratigraphic framework also sheds predictive light on facies distribution in the underexplored Cook Inlet basin. LST–1 may contain prospective hydrocarbon reservoir facies basinward of the study area and consists nearly exclusively of sandstone and conglomerate in outcrop. Four marine settings of LST–2 were prone to accumulating coarse-grained sediment and could host reservoir facies: shelf (inferred), deep-water canyon fill, canyon-associated master levee, and base of slope to basin floor. However, Naknek sandstones contain a high proportion of labile grains, resulting in diagenetic destruction of porosity and permeability. Nevertheless, we propose that compositional variability in the batholith in conjunction with sediment routing pathways to depocenters prone to coarse-grained sedimentation influenced whether and where conventional reservoir quality might occur in the basin. This study is relevant to exploration in Cook Inlet regardless of conventional reservoir quality parameters, as ubiquitous fracture networks in the Naknek may in the subsurface yield unconventional, fractured reservoir prospectivity.
The Naknek Formation exhibits remarkable tectono-stratigraphic similarities across its regional extent, likely reflecting convergent margin-scale factors that influenced the structural and depositional evolution of the arc–forearc region in southern Alaska during the Late Jurassic.

Lithostratigraphic (black lines and white labels) and sequence stratigraphic (red lines and labels) interpretation of the Naknek Formation at the northern extent of Chisik Island in Tuxedni Bay. Naknek member abbreviations: Jnc—Chisik Conglomerate; Jnss—lower sandstone; Jns—Snug Harbor Siltstone; Jnp—Pomeroy Arkose. Jcp—Paveloff Siltstone Member, Chinitna Formation. See abstract for sequence stratigraphic abbreviations. Red arrows mark onlapping stratal terminations of channel-form sediment bodies along canyon margins.


Speakers: Trystan M. Herriott and Marwan A. Wartes, Alaska Division of Geological & Geophysical: Paul L. Decker, Alaska Division of Oil and Gas

Trystan Herriott received B.S. (2005) and M.S. (2006) 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 stratigraphy, sedimentology, geochronology, volcanology, structural geology, and geologic mapping to unravel basin histories in northern, interior, and southern Alaska.

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