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Late n a i m o c Senonian o e N Early Paleocene Eocene Oligocene Miocene Pliocene Early Late Late Early Middle Middle 0 20 40 60 80 100 120 140 160 180 200 220 240 Talkeetna Kamishak Tuxedni Naknek Staniukovich Herendeen/ Nelchina Matanuska Kaguyak Saddle Mtn Mbr Unnamed West Foreland Hemlock Tyonek Sterling Beluga Tectonism h c o p E d o i r e P a r E Stratigraphy Depositional Environment ) a M ( e g A y r a i t r e T c i o z o n e C s u o e c a t e r C c i o z o s e M c i s s a r u J c i s s a i r T c r a c i n a e c O Shallow marine, Shallow marine mixed carbonates and siliclastics Shallow marine carbonate, chert, minor tuffs d o r P / e c r u o S Andesitic flows, volcaniclastics Marine to nonmarine siliclastics deep-water turbidites m s i r p y r a n o i t e r c c a f o h t w o r G c i f i c a P - a l u K n o i t c u d b u s e g d i r x e l p m o c y r a n o i t e r c c A l e v e l a e s e v o b a e v i t c a t l u a f y a B n i u r B s k c o d T C W f o n o i t a m u h x E c r a w o l l a h s f o n o i t a m u h x E s t o o r c r a Fluvial, lacustrine, coal swamp, alluvial fan m s i t a m g a m f o t e s n O ) c r a n r e d o m o t l a r t s e c n a ( d e t a i t i n i F R B - n o i t c u d b u s s a ? t s u r h t d e t a l e r S S S S S t a t u k a Y n o i s i l l o c g n i d l o F a t a r t s z C D. R Sitka Kodiak Seward Valdez Cordova Yakutat Anchorage CANADA ALASKA A D A N A C A K S A L A YUKON TERRITORY BRITISH COLUMBIA 0 0 50 50 100 100 200 KILOMETERS 200 MILES NORTH AMERICAN PLATE PACIFIC PLATE TAL K E ET NA FS WEST F O R K F A U L T DENALI FAULT SYSTEM BORDER RANGES FAULT SYSTEM FAIRWEATHER FAULT TRANSITION FAULT SYSTEM ALEUTIAN MEGATHRUST CONTACT FAULT SYSTEM RESU RRECTI ON F A ULT M E T S Y S T L U A F I L A N E D CH UGA C H - S T . ELI AS FAU LT SYST EM CONTACT FAULT SYSTEM FAIRWEATHER FAULT YA YA YA CG CG CG PE PE PE PE WR WR AX WR WR AX AX AX AX CG CG YT Czu Czu KCb KCb KCb KCb KCb KCb - Cretaceous & Cenozoic basinal deposits YT - Yukon-Tanana terrane(s) PE - Peninsular terrane WR - Wrangellia terrane AX - Alexander terrane u - undifferentiated terranes CG - Chugach terrane PW - Prince William terrane GR - Ghost Rocks Formation YA - Yakutat terrane Czu - Upper Cenozoic accreted rocks u u PW PW PW PE KCb KCb KCb BORDER RANGES FAULT SYS. u U.S. CANADA ALASKA Map location A’ A WR WR ² 50.8 mm/yr 63.5 mm/yr D A NG E R O U S R I V E R Z O N E PAM PLO NA Z O NE 10 sl 50 100 150 kilometers A’ A NW SE Aleutian Range Cook Inlet Kenai Gulf of Alaska Trench Pacific Plate North American Plate Age of Accretionary Prizm 1 - Early Mesozoic 2 - Late Mesozoic 3 - Early Cenozoic 4 - Late Cenozoic 1 2 3 4 BBF BRFS BBF - Bruin Bay Fault BRFS - Border Ranges Fault System LAKE C L AR K FAUL T GR CG? 55° 60° 155° 150° 145° 140° 135° Oceanic crust Continental crust KB IL IP TB KL PB CRB AV SB LC Soldotna Ninilchik Anchor Point HB CAST LE M OUNT A IN F AUL T BRUIN BAY F AUL T S YST EM Homer CD SA BI SEM Cook Inlet Basin Cook Inlet Basin AARB AARB AP A. 3.1 3.2 3.4 3.5 3.3 3.7 5 2.4 2.1 TUXEDNI BAY Chisik Island CHINITNA BAY Johnson River Red Glacier INISKIN BAY INISKIN PENINSULA Oil Bay Tuxedni Channel COOK INLET Slope Mountain Triangle Peak Saddle Mountain Lenore Hill 4 2 0 4 8 MILES 4 0 4 8 12 KILOMETERS SCALE APPROXIMATELY 1:125,000 N Stratigraphic Units CHINITNA FORMATION (Middle Jurassic)—Marine siltstone and sand- stone, with subordinate conglomerate. Each member hosts a coarse-grained basal suc- cession with probable delta associations. Regional layer-cake-like stacking, but com- plex stratigraphic architecture is expressed in mountain-scale exposures. Typically ~700 m thick, with each member of comparable thickness. See Herriott and Wartes (2014: doi: 10.14509/27305) and references therein for further description. Q Q Q Q Q Quaternary deposits Ts Ts Tertiary strata Igneous Units }{i Undifferentiated igneous rocks, Meso–Cenozoic Jp Jp Plutonic rocks, Jurassic Bruin Bay fault system Locality discussed on poster Chinitna Formation observation locality Upper Cretaceous strata Jn Jn Jn Jn Jn Ks Naknek Formation, Upper Jurassic Jcp Paveloff Siltstone Member Tonnie Siltstone Member Jt Jt Jt Jt Jt Jct Jct Jt Tuxedni Group, Middle Jurassic Jtk Jtk Jtk Jtk Talkeetna Formation, Lower Jurassic Marble, Triassic(?) 152.75° W 60.00° N 59.75° N 153.25° W ^m Jct EXPLANATION STUDY AREA Tonnie Peak Jcp Jcp Jcp 2.5 3.6 2.2 2.3 Jct Jct B. C. L F Qt Transitional Arc Undissected Arc Dissected Arc Recycled Orogen Basement Uplift T ransitional Continental Craton Interior Qm K P L i m i t o f D e t r i t a l M o d e s Increasing maturity/stability from continental block provenances Increasing ratio of plutonic to volcanic sources in magmatic arc provenances Provenance fields from Dickinson (1985; in Provenance of Arenites [p. 333–361]) Circum-Pacific Volcanoplutonic Suites Porosity (%) 0 2 4 6 8 Klinkenberg Permeability (md) 0.0001 0.001 0.01 0.1 1 ȝP ȝP ȝP ȝP ȝP ȝP Paveloff Siltstone Mbr.—Tonnie Peak area ĭ N PG 13PD024A Paveloff Siltstone Mbr.—East shore Oil Bay ĭ N PG $$ Paveloff Siltstone Mbr.—East shore Oil Bay ĭ N PG $$ Paveloff Siltstone Mbr.—South shore Chinitna Bay ĭ N PG 70+% Paveloff Siltstone Mbr.—East shore Oil Bay ĭ N PG $$ Paveloff Siltstone Mbr.—East shore Oil Bay ĭ N PG 13A027-323.8A Sandstones in Paveloff Siltstone Member consist large- ly of calcic plagioclase (average 47%) that have been partially to totally albitized and lesser amounts of inter- mediate volcanic rock fragments (average 19%). Some grains are euhedral plagioclase crystals, suggesting derivation from relatively unconsolidated crystal tuffs. Grain sizes range from coarse silt to fine-grained sand through most of the Paveloff, but coarse- and very coarse-grained sandstones occur at the member’s base. The sandstones are typically cemented by authi- genic chlorite and lesser amounts of heulandite, result- ing in poor reservoir quality. Some sandstones have substantial ferroan-calcite cement, with intergranular volumes approaching 40%, which suggests relatively early cementation. One coarse-grained sandstone from the south shore of Chinitna Bay (see photomicro- graph at above right and locality 5 on map) is oil stained (see section 5; also Wartes and Herriott [2015; doi: 10.14509/29533]) and cemented by extensive au- thigenic chlorite and patchy ferroan-calcite. This sample has nearly no porosity visible in thin section, and the hydrocarbon most likely resides in microporos- ity associated with the chlorite cement. oil stained 13TMH058B oil stained 13TMH058B oil stained . A PRELIMINARY SEQUENCE- STRATIGRAPHIC FRAMEWORK KEY LOCATION AND GEOLOGIC SETTING: INISKIN–TUXEDNI BAYS AREA ABSTRACT D. Dark-gray mudstone in the Middle Jurassic Red Glacier Formation (Tuxedni Group) crops out near Red Glacier north of Chinitna Bay; organic-rich marine shales of the lower Tuxedni Group are likely the principal sources of oil in Cook Inlet’s producing reservoirs, which, as noted above, are of Tertiary age. See Stanley et al. (2013: doi: 10.14509/24824 [p. 5–9]) and LePain et al. (2013: AAPG Memoir 104) for further information about the Red Glacier Formation. Photograph by R.G. Stanley. C. The siliciclastic record in Cook Inlet basin comprises more than 35,000 feet of Middle Jurassic through Creta- ceous marine strata and up to 25,000 feet of nonmarine Tertiary stratigra- phy. Oil and gas are produced from sandstones within the Tertiary sec- tion; however, the oil is sourced from the underlying Middle Jurassic Tux- edni Group (see also D) and/or the Tri- assic Kamishak Formation, whereas the gas—of biogenic origin—is sourced from Tertiary coals. Red S indicates gas source rock; green S indicates oil source rock; red asterisk represents gas production; green circle represents oil production. Figure modified after Gillis (2013: doi: 10.14509/24824 [p. 1–4]) and refer- ences therein. 3.1 Oblique aerial view eastward of Paveloff Siltstone Member north- east of Johnson River in the Slope Mountain area. A thick, channelized succession at the base of Paveloff is well developed and clearly ex- pressed at this locality and dominantly comprises sandstone (see Jcp 1 ). Note high-relief erosional surface that cuts Jcp 2 and is filled by slumped, channelized, and tabular-bedded packages of Jcp 3 . Photo- graph by M.A. Wartes. 3.2 Oblique aerial view southward of tabular-bedded (lower part) to channelized (upper part) Jcp 1 within cliff-face exposure ~1.5 km west of Triangle Peak. Channel fills are sandstone and conglomerate and commonly >10 m thick. Convolute-stratified sandstones are sugges- tive of rapid dewatering in a high-sedimentation-rate setting. Herriott et al. (2016: doi: 10.14509/ 29539) interpreted Jcp 1 here as likely record- ing deltaic processes. Photograph by T.M. Herriott. 3.3 Oblique aerial view southeastward of Paveloff in the Red Glacier area northwest of Lenore Hill. Stratigraphic architecture here is similar to the exposure at 3.1, which lies ~12 km to the northeast. Note, how- ever, greater relief along the Jct–Jcp contact here than is observed at 3.1, 3.2, or 3.7, and the seemingly sand-rich nature (see lighter-brown- weathering strata) of lowermost Jcp 3 . Two scenarios regarding the genesis of the basal Jcp 3 surface are considered below. Photograph by M.A. Wartes. See Herriott et al. (2016: doi: 10.14509/29539) for further discussion of 3.1 and 3.2. Subscripted architectural units are further described in 3.7. 3.4 Oblique aerial view southwestward of Tonnie Siltstone Member in the Triangle Peak area. Jct 1 is a sand-rich succession that sharply overlies Jt (see also 3.7). Note fining-upward trend at base of Jct 2 and onlap of Jct 3 strata onto a low-relief erosional surface. Tonnie is capped by a coarsening- and thickening-upward succession with local channel forms. Upper part of exposure is visible in 3.2. Photograph by T.M. Herriott. 3.5 Oblique aerial view eastward of lower Tonnie southwest of Red Glacier. Note channel-form stratal geometries in this sandstone, conglomerate, and siltstone section, and a relatively high-relief erosional surface at the Jt–Jct contact (compare to 3.7). The Jct 1 –Jct 2 transition is mapped at an inferred fining- and thinning-upward transition (compare with 3.4 and 3.7). Photograph by T.M. Herriott. An oil-stained locality within lower Paveloff was discovered during a geologic mapping traverse along the south shore of Chinitna Bay (below). The hydrocarbon-bearing zone occurs immediately above the base of what is recognizable as Jcp 1 and comprises very-thick-bedded, structureless to faintly stratified, coarse-grained sand- stone (left and right) with common “floating” granules (right). Sedimentary textures and stratification of the interval suggest deposition from dense, laminar flows was common. Wartes and Herriott (2015: doi: 10.14509/29533) preliminarily interpreted these strata as delta-front deposits that accumulated during an episode of high sediment supply. Reservoir quality parameters of the oil-stained sample are among the best known from Paveloff (see section 4), suggesting that down-dip equiv- alents to Jcp 1 may host accumulations of oil and should be considered within the context of Mesozoic play concepts in Cook Inlet. Photographs by T.M. Herriott. A ~70-m-thick succession of channelized Chinitna Forma- tion crops out at the north end of Chisik Island. Matrix- to clast-supported cobble and boulder conglomerates are common, as are very thick beds of structureless sand- stone. Channel-fills up to 6 m thick and 25 m wide are ob- served. Sedimentary textures and stratification suggest high energy, laminar to turbu- lent flows deposited the coarse sediment, and marine fossils occur in the outcrop. Tens of m of incision into the underlying unit (Jt or Jct; see 3.6) is consistent with the sediment supply regime. We tentatively interpret these strata as shelf-valley-fill de- posits. Photographs by T.M. Herriott. Similar to Paveloff, observations of sedimentary textures, structures, bedding, and fossils throughout Tonnie Siltstone Member are consistent with shallow-marine sedimentation in delta, prodelta, and outer-shelf settings. The generally thin-bedded, very-fine-grained sandstone and siltstone (above left) of the upper ~200 m of Tonnie is well exposed at Iniskin Bay (below left). Trace fossils (e.g., below right: Planolites[?]), body fossils (e.g., ammonite at left), and thin-bedded and sharp-based sandstones (above right) are suggestive of prodelta and/or outer-shelf(?) settings that may have been prone to storm- influenced sedimentation. Fossil and plant debris in pot-cast-like features (left) may be biogenic collections. The upper part of Tonnie at Iniskin bay (below left) hosts a gully-scale cut-and-fill element —although the stratigraphic relations are somewhat obscured by faulting—that may be associated with highstand progradation (see also 3.4 and 3.7). Photographs by T.M. Herriott. The Chinitna Formation of lower Cook Inlet is a ~700-m-thick marine unit that crops out near the arc-proximal forearc basin margin. Two members of comparable thickness are mapped as Tonnie Siltstone (Bathonian–Callovian) and Paveloff Siltstone (Callovian). Geologic mapping, stratigraphic reconnaissance, and sedimentologic work provide new insights into the Chinitna. A ~70-m-thick channelized conglomerate package at Chisik Island is reportedly associated with Tonnie, but field relations indicate these beds may be younger than Tonnie. Nevertheless, lower Tonnie exposures near Tonnie Peak do host channelized, cross-stratified sandstone. At Iniskin Bay, part of lower Tonnie exhibits thin, sharp-based sandstone intercalated with bioturbated siltstone; hummocky cross-stratified sandstone is also present. Reconnaissance of upper Tonnie reveals a finer-grained interval with local gully-scale channel-forms and mainly fine-grained fills. A generally comparable stratigraphic stacking motif is documented in Paveloff. A ~100-m-thick succession of tabular and channelized sandstone and conglomerate commonly occurs at this member’s base, and hummocky cross stratification is also noted. Regionally, overlying finer-grained deposits are observed. Upper Paveloff at Chinitna Bay comprises more than 160 m of bioturbated, very fine-grained sandstone with subordinate coarser, sharp-based sandstone; slump scars and channels with m-scale relief are principally filled with fine-grained detritus. Mountain-scale exposures exhibit even larger channel-forms in upper Paveloff, including a slump-associated feature with ~140 m of stratigraphic relief. Tonnie and Paveloff each record third-order sedimentation cycles. Regressive, lowstand depositional systems with probable delta associations supplied coarse sediment during onset of each cycle. The conglomerate at Chisik Island highlights marked base-level fall (10s of m of incision), probably represents shelf-valley fill, and is tentatively associated with Paveloff rather than Tonnie. Overlying finer-grained successions in both members may reflect waning deltaic influences as near-shore environments were transgressed during rising base level, diminishing sediment supply to prodelta settings in shelfal water depths ranging down to—and perhaps below—storm wave base. Continued transgressions likely terminated direct deltaic inputs into outer shelf settings. The lithologically monotonous, gullied upper parts of each member may record highstand normal regressions— rather than continued transgressions—when muddy clinoforms(?) of delta- to slope-scale relief prograded into the basin during later periods of base-level rise; the strongest evidence for this scenario occurs in Paveloff, where the largest channel-form approaches submarine- canyon-scale. Rock-Eval pyrolysis results from 44 samples (12 from Tonnie, 32 from Paveloff) indicate poor petroleum source potential, with total organic carbon values of 0.14–0.69 weight percent and S2 values of 0.00–0.57 milligrams hydrocarbon per gram of rock. Thermal maturity of the samples ranges from ~0.7% Ro at Oil Bay to 0.85–1.20% Ro at Iniskin Bay based on Rock-Eval Tmax, spore color, and vitrinite reflectance analyses. Sampled Chinitna sandstones are mainly feldspathic and generally have less than 6% porosity and less than 0.2 millidarcies permeability. Nevertheless, migrated oil is documented in a lower Paveloff outcrop, and viable scenarios exist for Chinitna-hosted oil accumulations. SEDIMENTOLOGY 2 STRATIGRAPHIC ARCHITECTURE CHINITNA FORMATION OVERVIEW—MAP -SCALE ARCHITECTURAL UNITS AND SEQUENCE STRATIGRAPHY CHISIK IS.: T ONNIE or P AVELOFF ? CONGLOMERATE AT CHISIK ISLAND T ONNIE SILTSTONE MEMBER EXPLANATION P AVELOFF SILTSTONE MEMBER P AVELOFF SILTSTONE MEMBER 3 3.7 2.5 T ONNIE SILTSTONE MEMBER 2.4 OIL-STAINED OUTCROP: PAVELOFF 5 5 1 PETROLOGY AND RESERVOIR QUALITY: PAVELOFF 4 CONTEXT, HIGHLIGHTS, AND RESULTS Oil production in Cook Inlet forearc basin is from Tertiary reservoirs Oil source rocks in the basin occur in the Middle Jurassic Tuxedni Group and/or Triassic stratigraphy Does the Middle Jurassic Chinitna Formation have oil reservoir potential? Chinitna Formation stratigraphic cyclicity reflects at least two episodes when coarse detritus was ex- ported into the basin and should be recognized in the context of Mesozoic play concepts for Cook Inlet The depositional-systems and sequence-stratigraphic framework of this study—and an oil-stained outcrop—demonstrate that viable scenarios exist for oil reservoirs in the Chinitna Formation Chinitna Formation comprises ~700 m of dominantly fine-grained marine forearc basin strata Sedimentologic work suggests primarily shallow-marine deposition, but there are indications that some Chinitna strata may record deep-water (for example, slope) sedimentation Mountain-scale exposures reveal stratigraphic cycles in the Chinitna Formation Coarse-grained basal successions in each member are lowstand systems tracts Coarse detritus was exported beyond the outcrop belt during these lowstands Finer-grained middle and upper parts of each member are transgressive and highstand systems tracts, although stratigraphic relations within mid-Paveloff may reflect an additional base-level cycle Paveloff sandstones are feldspathic and generally have <6% porosity and <0.2 millidarcies permeability Oil-stained basal Paveloff (Jcp 1 ) crops out at Chinitna Bay 1 2 3 4 5 Jt Jct 2 Jct 4 Jct 4 Jct 4 Jct 3 Jct 3 Jct 1 Jcp 1 ~95 m ~95 m ~90 m Jcp 1 Jcp 1 Jcp 1 Jcp 4 Jcp 2 Jcp 2 Jcp 3 Jct 4 Jct 3 Jcp 1 Jcp 4 Jn Jt Jcp 3 Jn Jn Jn Jn—Naknek Formation Jcp—Paveloff Siltstone Member, Chinitna Formation Jct—Tonnie Siltstone Member, Chinitna Formation Jt—Tuxedni Group ~340 m ? ? ? Jcp Jcp Jct Jct or Jt? Jcp 1 or Jct 1 ? Chisik Island summit area Tuxedni Channel Jt Jct 1 Jt Jct 1 Jct 2 Jct 2 Jct 4 Jct 3 Triangle Peak Saddle Mountain Saddle Mountain formation/member contact intra-member contact large-scale erosional surface with onlap at arrow channel-form base deformed stratal surface/ zone of chaotic bedding fault—arrow indicates relative stratigraphic offset disrupted perspective of stratigraphy approximate stratigraphic thickness Jcp 1 Jcp 1 Jcp 4 Jcp 2 Jcp 3 Jct Jn Jn Jcp 1 Jcp 4 Jcp 2 Jcp 3 Jct Jct ~105 m ~55 m ~140 m of erosional relief on this surface ? ? ? ? ? COOK INLET STRATIGRAPHY The long-lived Cook Inlet forearc basin of south-central Alaska lies be- tween the Bruin Bay and Border Ranges fault systems that are bordered, respectively, by the Aleutian–Alaska Range batholith (AARB) to the north- west and an emergent accretionary prism (CG) to the southeast (see A [from LePain et al., 2013: AAPG Memoir 104]). This presentation focuses on the Middle Jurassic Chinitna Formation, which is well exposed along an outcrop trend between Iniskin and Tuxedni bays of lower Cook Inlet. We made observations of the Chinitna Formation at ~350 localities since our work in lower Cook Inlet began in 2009. Geologic mapping (B) southwest of Johnson River is preliminary and simplified (DGGS, unpublished data); farther northeast, mapping is after Detterman and Hartsock (1966: USGS Professional Paper 512), and is an area where DGGS and collaborators will map the geology during summer 2017. ~1m ? ? ? Jct 1 Jct 2 2.3 3.2 Paveloff Tonnie Basal Surface of Forced Regression SEQUENCE BOUNDARY SEQUENCE BOUNDARY Basal Surface of Forced Regresssion Transgressive Surface TRANSGRESSIVE SYSTEMS TRACT LOWSTAND SYSTEMS TRACT Maximum Flooding Surface Jcp 2 Jcp 1 Jcp Scenario 1 Jcp Scenario 2 Jcp 4 Jn Jcp 3 Paveloff Tonnie Jcp 2 Jcp 2 Jcp 1 Jcp 4 Jcp 3 Jcp 2 Jcp 2 HIGHSTAND SYSTEMS TRACT *Systems tracts after Posa- mentier and Allen (1999: SEPM Concepts in Sedimen- tology and Paleontology #7) Jct 4 —Coarsening- and thickening-upward succession of sandstone and siltstone. Sandstones locally channelized but domi- nantly tabular. Highstand deposits of possi- bly prodelta and delta settings. Jct 3 —Siltstone with subordinate sandstone. Locally onlaps Jct 2 along low-relief surface that may have formed via increased energy flux in outer(?)-shelf setting at onset of highstand regression. Maximum flooding surface likely at or below contact with Jct 2 . Jct 2 —Thick, fining-upward succession of mainly siltstone. Lower part may record waning prodelta sedimentation as Jct 1 deltas were transgressed. Upper part may reflect outer-shelf sedimentation during continued transgression. Interval’s base identified as transgressive surface. Jct 1 —Sandstone and conglomerate with subordinate siltstone. Interval is channel- ized in part. Unit’s sharp base locally exhib- its 10s of m erosional relief and is a sequence-bounding basal surface of forced regression. Deltaic(?) deposits, and may include shelf-valley-fill strata. Photograph by T.M. Herriott. Jcp 4 —Coarsening-upward, tabular succes- sion of siltstone and sandstone. Highstand regressive (prodelta[?] and delta[?]) deposits (Scenario 1), but may also include transgres- sive strata (Scenario 2). Capped by regionally significant sequence boundary (Herriott et al. [2017: doi: 10.14509/29707]). Jcp 3 —Slumped, channelized, and tabular succession of siltstone and sandstone that fill 100+ m of erosional relief cut into, and locally through, Jcp 2 . Base tentatively associated with mass-wasted clinoform foresets of delta- to slope-scale relief during highstand regres- sion (Scenario 1; see Herriott et al. [2016: doi: 10.14509/29539]). Base alternatively records allogenically forced base-level fall along basal surface of forced regression (Scenario 2). Jcp 2 —Thick succession of siltstone and sand- stone(?). Lower part comparable to transgres- sive Jct 2 . Upper part likely comprises shelfal, highstand regressive strata. Cryptic maxi- mum flooding surface is intra-Jcp 2 . Jcp 1 —Dominantly channelized sandstone and conglomerate. Sharp, typically planar base is basal surface of forced regression. Probable delta-associated deposits. Lower Tuxedni Group in outcrop: Probable equivalents to basin’s oil source rocks 3.6 3.6 Oblique aerial view eastward of the channelized, channel-form-hosted conglomerate at Chisik Island (see also 2.5). Fieldwork during summer 2017 will aim to determine whether these shelf-valley(?) strata are Jct 1 or Jcp 1 , and biostratigraphic and/or geochronologic constraints may be key to resolving these enigmatic stratigraphic relations. Photograph by T.M. Herriott Sedimentary textures, structures, bedding character and geometries, and fossil assemblages observed throughout Paveloff Siltstone Member are principally consistent with shallow-marine sedimentation in delta, prodelta, and outer-shelf settings. Some exposures clearly indicate rela- tively steep depositional gradients, potentially reflecting delta-scale clinoform foresets; however, deep-water pro- cesses associated with slope-scale clinoforms may also occur in the Paveloff (see 3.7). 2.1 Detailed study of upper Paveloff at Chinitna Bay reveals a thick (~160 m), locally slumped (above left) succession of chiefly lithologically monotonous, thoroughly bio- turbated, very-fine-grained sandstone and siltstone with locally discrete Phycosiphon trace fossils (above right); subordinate thin beds of fine-grained sandstone are com- monly ripple laminated (right). Photographs by M.A. Wartes. 2.2 Excellent exposures of lower Paveloff occur in the Tonnie Peak area, where probable hummocky cross-stratified sandstones are observed. Photographs by P.L. Decker. 2.3 Detailed study of Paveloff in Oil Bay indicates a prodelta setting within the member. Thin, sharp-based sandstone beds are sediment gravity flow deposits, which are locally bioturbated (see example of Thalassinoides to right of pencil tip). Pho- tograph by T.M. Herriott. CHINITNA 2.3 SEE 2.3 SEE SEE 5 2.3 SEE 2.3 SEE 2.2 SEE ~140 m ~270 m Authors’ note: Subsequent field observations made during summer 2017 suggest that this conglomerate succession comprises Tonnie Siltstone Member of Jct 1 affinity, lying above an incision cut into uppermost Jt strata. 2.2 2.1 3.1 3.3 3.4 3.5 STRATIGRAPHY AND SEDIMENTOLOGY OF THE CHINITNA FORMATION, INISKIN–TUXEDNI BAYS AREA, SOUTH-CENTRAL ALASKA—LATE MIDDLE JURASSIC DEPOSITIONAL SYSTEMS AND PETROLEUM PROSPECTIVITY IN COOK INLET FOREARC BASIN Trystan M. Herriott 1 , Marwan A. Wartes 1 , Richard G. Stanley 2 , Paul L. Decker 3 , Kenneth P. Helmold 3 , and Nina T. Harun 1 1 Alaska Division of Geological & Geophysical Surveys, Fairbanks, AK ([email protected]) 2 U.S. Geological Survey, Menlo Park, CA; 3 Alaska Division of Oil and Gas, Anchorage, AK DIVISION OF OIL AND GAS Alaska Department of RESOURCES NATURAL
