Document a1EGZ220j06Kpk7w4BzbKGd7b

Conversation Contents NPR-A Map for Today's Secretary Briefing Attachments: /71. NPR-A Map for Today's Secretary Briefing/1.1 LeaseTracts_Suggested_RecentDiscoveries (Zinke Briefing 30May2017).jpg /71. NPR-A Map for Today's Secretary Briefing/3.1 2009 Houseknecht et al Clinoform Sequences.pdf /71. NPR-A Map for Today's Secretary Briefing/3.2 Nanushuk-Torok seismic facies map.pdf "Gieryic, Michael" <mike.gieryic@sol.doi.gov> From: Sent: To: Subject: Attachments: "Gieryic, Michael" <mike.gieryic@sol.doi.gov> Tue May 30 2017 10:15:21 GMT-0600 (MDT) David Houseknecht <dhouse@usgs.gov> NPR-A Map for Today's Secretary Briefing LeaseTracts_Suggested_RecentDiscoveries (Zinke Briefing 30May2017).jpg See light green "pencil sticks". Mike Gieryic Attorney-Adviser Office of the Regional Solicitor U.S. Department of the Interior 4230 University Drive, Suite 300 Anchorage, AK 99508 Phone: (907) 271-1420; Fax: (907) 271-4143 mike.giervic@.sol.doi.gov "Houseknecht, David" <dhouse@usgs.gov> From: Sent: To: Subject: "Houseknecht, David" <dhouse@usgs.gov> Tue May 30 2017 10:19:02 GMT-0600 (MDT) "Gieryic, Michael" <mike.gieryic@sol.doi.gov> Re: NPR-A Map for Today's Secretary Briefing Yes, those green lines look familiar! Thanks for sending. On Tue, May 30, 2017 at 12:15 PM, Gieryic, Michael <mike.giervic@sol.doi.gov> wrote: See light green "pencil sticks". Mike Gieryic Attorney-Adviser Office of the Regional Solicitor U.S. Department of the Interior 4230 University Drive, Suite 300 Anchorage, AK 99508 Phone: (907) 271-1420; Fax: (907) 271-4143 mike.giervic@sol.doi.gov Dave Houseknecht U.S. Geological Survey 12201 Sunrise Valley Drive MS 956 Reston, VA 20192 (703) 648-6466 "Houseknecht, David" <dhouse@usgs.gov> From: Sent: To: Subject: Attachments: "Houseknecht, David" <dhouse@usgs.gov> Tue May 30 2017 11:43:51 GMT-0600 (MDT) "Gieryic, Michael" <mike.gieryic@sol.doi.gov> Re: NPR-A Map for Today's Secretary Briefing 2009 Houseknecht et al Clinoform Sequences.pdf NanushukTorok seismic facies map.pdf Mike: Attached are (1) updated map of Nanushuk-Torok seismic facies, and (2) paper published on these rocks in 2009. On #1, the red blobs are (west to east) Smith Bay, Willow, and Pikka discoveries. The greatest potential occurs along the blue dashed lines (lowstand shelf margins in geologic jarbon), east of the red dashed line, and north of ~69.5 degrees north. The vertical (N-S) green line on the BLM map is accurate, but I would move the horizontal green line farther south. Cheers, On Tue, May 30, 2017 at 12:15 PM, Gieryic, Michael <mike.giervic@.sol.doi.gov> wrote: See light green "pencil sticks". Mike Gieryic Attorney-Adviser Office of the Regional Solicitor U.S. Department of the Interior 4230 University Drive, Suite 300 Anchorage, AK 99508 Phone: (907) 271-1420; Fax: (907) 271-4143 mike.giervic@.sol.doi.gov U.S. Department of the Interior 4230 University Drive, Suite 300 Anchorage, AK 99508 Phone: (907) 271-1420; Fax: (907) 271-4143 mike.giervic@sol.doi.gov Dave Houseknecht U.S. Geological Survey 12201 Sunrise Valley Drive MS 956 Reston, VA 20192 (703) 648-6466 "Houseknecht, David" <dhouse@usgs.gov> From: Sent: To: Subject: "Houseknecht, David" <dhouse@usgs.gov> Wed Jun 07 2017 10:41:02 GMT-0600 (MDT) "Gieryic, Michael" <mike.gieryic@sol.doi.gov> Re: NPR-A Map for Today's Secretary Briefing I will send to BLM and BOEM (Anchorage) this afternoon a set of discussion points and ask for a conference call either tomorrow or Friday regarding this topic. Would it be easier to include you in that, or do you want to chat today? On Wed, Jun 7, 2017 at 12:05 PM, Gieryic, Michael <mike.giervic@.sol.doi.gov> wrote: Dave, Please give me a call to discuss section 4(b) of Secretarial Order 3352. Mike Gieryic Attorney-Adviser Office of the Regional Solicitor U.S. Department of the Interior 4230 University Drive, Suite 300 Anchorage, AK 99508 Phone: (907) 271-1420; Fax: (907) 271-4143 mike.giervic@sol.doi.gov On Tue, May 30, 2017 at 9:43 AM, Houseknecht, David <dhouse@usgs.gov> wrote: Mike: Attached are (1) updated map of Nanushuk-Torok seismic facies, and (2) paper published on these rocks in 2009. On #1, the red blobs are (west to east) Smith Bay, Willow, and Pikka discoveries. The greatest potential occurs along the blue dashed lines (lowstand shelf margins in geologic jarbon), east of the red dashed line, and north of ~69.5 degrees north. The vertical (N-S) green line on the BLM map is accurate, but I would move the horizontal green line farther south. Cheers, On Tue, May 30, 2017 at 12:15 PM, Gieryic, Michael <mike.gieryic@sol.doi.gov> wrote: See light green "pencil sticks". Mike Gieryic Attorney-Adviser Office of the Regional Solicitor U.S. Department of the Interior 4230 University Drive, Suite 300 Anchorage, AK 99508 Phone: (907) 271-1420; Fax: (907) 271-4143 mike.gieryic@sol.doi.gov Dave Houseknecht U.S. Geological Survey 12201 Sunrise Valley Drive MS 956 Reston, VA 20192 (703) 648-6466 Dave Houseknecht U.S. Geological Survey 12201 Sunrise Valley Drive MS 956 Reston, VA 20192 (703) 648-6466 "Gieryic, Michael" <mike.gieryic@sol.doi.gov> From: Sent: To: Subject: "Gieryic, Michael" <mike.gieryic@sol.doi.gov> Wed Jun 07 2017 11:11:41 GMT-0600 (MDT) "Houseknecht, David" <dhouse@usgs.gov> Re: NPR-A Map for Today's Secretary Briefing I was hoping to get a quick sense of what you have been tasked with, regarding putting together a plan that addresses section 4b of the Secretarial Order, so that we (BLM-AK and I) are not duplicating efforts as we draft a response to the order. My sense is that would be a 5 minute conversation. Mike Gieryic Attorney-Adviser Office of the Regional Solicitor U.S. Department of the Interior 4230 University Drive, Suite 300 Anchorage, AK 99508 Phone: (907) 271-1420; Fax: (907) 271-4143 mike.giervic@sol.doi.gov On Wed, Jun 7, 2017 at 8:41 AM, Houseknecht, David <dhouse@usgs.gov> wrote: I will send to BLM and BOEM (Anchorage) this afternoon a set of discussion points and ask for a conference call either tomorrow or Friday regarding this topic. Would it be easier to include you in that, or do you want to chat today? On Wed, Jun 7, 2017 at 12:05 PM, Gieryic, Michael <mike.gieryic@sol.doi.gov> wrote: Dave, Please give me a call to discuss section 4(b) of Secretarial Order 3352. Mike Gieryic Attorney-Adviser Office of the Regional Solicitor U.S. Department of the Interior 4230 University Drive, Suite 300 Anchorage, AK 99508 Phone: (907) 271-1420; Fax: (907) 271-4143 mike.gieryic@sol.doi.gov On Tue, May 30, 2017 at 9:43 AM, Houseknecht, David <dhouse@usgs.gov> wrote: Mike: Attached are (1) updated map of Nanushuk-Torok seismic facies, and (2) paper published on these rocks in 2009. On #1, the red blobs are (west to east) Smith Bay, Willow, and Pikka discoveries. The greatest potential occurs along the blue dashed lines (lowstand shelf margins in geologic jarbon), east of the red dashed line, and north of ~69.5 degrees north. The vertical (N-S) green line on the BLM map is accurate, but I would move the horizontal green line farther south. Cheers, On Tue, May 30, 2017 at 12:15 PM, Gieryic, Michael <mike.giervic@.sol.doi.gov> wrote: See light green "pencil sticks". Mike Gieryic Attorney-Adviser Office of the Regional Solicitor U.S. Department of the Interior 4230 University Drive, Suite 300 Anchorage, AK 99508 Phone: (907) 271-1420; Fax: (907) 271-4143 mike.gieryic@sol.doi.gov On Wed, Jun 7, 2017 at 12:05 PM, Gieryic, Michael <mike.gieryic@sol.doi.gov> wrote: Dave, Please give me a call to discuss section 4(b) of Secretarial Order 3352. Mike Gieryic Attorney-Adviser Office of the Regional Solicitor U.S. Department of the Interior 4230 University Drive, Suite 300 Anchorage, AK 99508 Phone: (907) 271-1420; Fax: (907) 271-4143 mike.gieryic@sol.doi.gov On Tue, May 30, 2017 at 9:43 AM, Houseknecht, David <dhouse@usgs.gov> wrote: Mike: Attached are (1) updated map of Nanushuk-Torok seismic facies, and (2) paper published on these rocks in 2009. On #1, the red blobs are (west to east) Smith Bay, Willow, and Pikka discoveries. The greatest potential occurs along the blue dashed lines (lowstand shelf margins in geologic jarbon), east of the red dashed line, and north of ~69.5 degrees north. The vertical (N-S) green line on the BLM map is accurate, but I would move the horizontal green line farther south. Cheers, On Tue, May 30, 2017 at 12:15 PM, Gieryic, Michael <mike.gieryic@sol.doi.gov> wrote: See light green "pencil sticks". Mike Gieryic Attorney-Adviser Office of the Regional Solicitor U.S. Department of the Interior 4230 University Drive, Suite 300 Anchorage, AK 99508 Phone: (907) 271-1420; Fax: (907) 271-4143 mike.gieryic@sol.doi.gov Dave Houseknecht U.S. Geological Survey 12201 Sunrise Valley Drive MS 956 Reston, VA 20192 (703) 648-6466 Basin Research (2009) 21, 644-654, doi: 10.1111/j.1365-2117.2008.00392.x Seismic analysis of clinoform depositional sequences and shelf-margin trajectories in Lower Cretaceous (Albian) strata, Alaska North Slope David W. Houseknecht*, Kenneth J. Birdw and Christopher J. Schenk *US Geological Survey, Reston, VA, USA wUS Geological Survey, Menlo Park, CA, USA zUS Geological Survey, Denver, CO, USA ABSTRACT Lower Cretaceous strata beneath the Alaska North Slope include clinoform depositional sequences that filled the western Colville foreland basin and overstepped the Beaufort rift shoulder. Analysis of Albian clinoform sequences with two-dimensional (2D) seismic data resulted in the recognition of seismic facies inferred to represent lowstand, transgressive and highstand systems tracts. These are stacked to produce shelf-margin trajectories that appear in low-resolution seismic data to alternate between aggradational and progradational. Higher-resolution seismic data reveal shelf-margin trajectories that are more complex, particularly in net-aggradational areas, where three patterns commonly are observed: (1) a negative (downward) step across the sequence boundary followed by mostly aggradation in the lowstand systems tract (LST), (2) a positive (upward) step across the sequence boundary followed by mostly progradation in the LSTand (3) an upward backstep across a mass-failure decollement.These different shelf-margin trajectories are interpreted as (1) fall of relative sea level below the shelf edge, (2) fall of relative sea level to above the shelf edge and (3) massfailure removal of shelf-margin sediment. Lowstand shelf margins mapped using these criteria are oriented north- south in the foreland basin, indicating longitudinal filling from west to east. The shelf margins turn westward in the north, where the clinoform depositional system overstepped the rift shoulder, and turn eastward in the south, suggesting pro gradation of depositional systems from the ancestral Brooks Range into the foredeep. Lowstand shelf-margin orientations are consistently per pendicular to clinoform-foreset- dip directions. Although the Albian clinoform sequences of the Alaska North Slope are generally similar in stratal geometry to clinoform sequences elsewhere, they are significantly thicker. Clinoform-sequence thickness ranges from 600-1000 m in the north to 1700-- 2000 m in the south, reflecting increased accommodation from the rift shoulder into the foredeep. The unusually thick clinoform sequences suggest significant subsidence followed by rapid sediment influx. INTRODUCTION Petroleum exploration in the Alaska North Slope (Fig. 1) during the past decade has focused increasingly on strati graphic traps, including objectives in clinoform strata of the Cretaceous-Tertiary Brookian sequence (Fig. 2). Ex ploration targets have included sandstone reservoirs in both the lower and upper parts of clinoforms, interpreted as deep-marine (toe-of-foreset) and shallow-marine to non-marine (topset) facies, respectively. Several oil discov eries have been developed in Upper Cretaceous and Pa laeogene reservoirs and, most recently, in Lower Cretaceous reservoirs of both deep- and shallow-water fa cies that have been developed as satellite pools (Nanuq Correspondence: David W. Houseknecht, U.S. Geological Sur vey, Reston,VA, USA. E-mail: dhouse@usgs.gov and Qannik pools, respectively, Fig. 3) within the larger Al pine oil field, whose main reservoir is a Jurassic shoreface sandstone (Houseknecht & Bird, 2004). The development of the Nanuq and Qannik pools, together with the occur rence of oil-stains in outcrops of lower-slope sandstone facies (Houseknecht & Schenk, 2007), demonstrate the ex ploration potential in Lower Cretaceous clinoforms be neath the western North Slope and Chukchi Sea (Fig. 1). Although the regional stratigraphic framework of the Lower Cretaceous clinoforms is well known, few pub lished reports have interpreted the sequence stratigraphy of these strata.The objectives of this paper are to describe the stratal geometry and shelf-margin trajectories ofLower Cretaceous clinoform depositional sequences in the eastern part of the National Petroleum Reserve in Alaska (NPRA) and adjacent areas (Fig. 1) and to interpret their sequence stratigraphy. No claim to original US government works 644 Journal Compilation Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association ofSedimentologists Seismic analysis of clinoform depositional sequences, Alaska S.MT non-deposition and Paleogene \ truncation Chukchi Sea northern margin of Beaufort rift shoulder Canada Basin Lower Cretaceous strata presumed present north of rift shoulder Seismic Facies I Proximal I Clinoform Slope wedge i I Distal condensed Clinoform Paleocurrents Foreset Dip Topset X-beds NPRA northern limit ANWR Hope Basin Fig. 1. Map of the Alaska North Slope (onshore area north of Brooks Range), Chukchi Sea and Beaufort Sea showing main geologic elements, seismic facies and palaeocurrent data from Lower Cretaceous first-order depositional sequence, study area shown in Fig. 3, and exploration wells (black dots). The Lower Cretaceous sequence is presumed to be present north of the Beaufort rift shoulder, but is buried too deeply to resolve with available seismic data. Outcrops of this sequence occur between the northern front of the Brooks Range and the dashed line labelled `northern limit of outcrops'. Note that sequence is partly to completely absent in the north due to non-deposition on part of Beaufort rift shoulder, truncation by Late Cretaceous incision and truncation across Palaeogene uplift. Ultimate shelf margin of Lower Cretaceous depositional sequence is indicated by the solid line between clinoform and slope-wedge seismic facies. Foreset dip directions are based on original work and work by Bird & Andrews (1979); topset cross-bed directions are from Huffman etal. (1985). Federal boundaries shown include the National Petroleum Reserve in Alaska (NPRA), the Arctic National Wildlife Refuge (ANWR), and the ANWR 1002 area. Hope basin is aTertiary successor basin unrelated to this study. GEOLOGIC SETTING The tectonics ofArctic Alaska during the Early Cretaceous included the simultaneous development of the Beaufort rift shoulder on the north (rift opening of the Canada ba sin), the Brooks Range orogenic belt on the south, and the Herald arch thrust belt on the west (Fig. 1). The Colville foreland basin formed in response to tectonic loading by the ancestral Brooks Range and Herald arch (Bird & Molenaar, 1992; Moore etal., 1994). The Lower Cretaceous fill of the Colville foreland basin extends from the Chukchi plat form, a pre-Mississippian ancestral high whose axis is co incident with the Russia - United States maritime boundary, to the southeastern North Slope, where it was mostly eroded during Tertiary uplift of the eastern Brooks Range (Fig. 1). The Brookian sequence (Fig. 2) includes Lower Cretac eous through Tertiary strata comprising sediment derived from the Brooks Range and tectonic uplands in eastern Siberia (west of the map in Fig. 1) and deposited in the fore land basin, on the Beaufort rift shoulder, and in the Canada basin north of the rift shoulder (Bird & Molenaar, 1992; Moore etal., 1994; Houseknecht etal., 2009).With the rift shoulder acting as an accommodation sill, filling ofthe Colville foreland basin generally progressed from west to east as indicated by the age and stratal geometry of the basin fill (Molenaar, 1983, 1988; Bird & Molenaar, 1992; Houseknecht et al, 2009). Within the Colville foreland basin, Lower Cretaceous clinoforms and their component formations have been documented in studies of regional stratigraphy (Molenaar, 1983,1985,1988; Bird & Molenaar, 1992), sequence stratigraphy (McMillen, 1991; House knecht & Schenk, 2001), petroleum geology (Bird, 1985, 2001), submarine mass-wasting (Weimer, 1987; Homza, 2004) and stratal geometry of source rocks (Creaney & Passey, 1993). This paper is focused on clinoform depositional se quences that cross formation boundaries of the Hue Shale, Torok Formation and Nanushuk Formation (Fig. 2). These clinoform strata are mostly of Albian age in the study area (Mull etal, 2003) and range into the Cenomanian in their easternmost extent. The Hue Shale is a mudstone that ac cumulated in the most distal part of the depositional sys tem. This mudstone includes oil-prone source rocks, mostly in highly condensed beds that include the infor mally named `gamma-ray zone' (GRZ; also known as the `highly radioactive zone' or HRZ) (Carman & Hardwick, 1983; Molenaar, 1983,1985,1988; Molenaar etal., 1987; Bird & Molenaar, 1992; Creaney & Passey, 1993). The Hue Shale is expressed as bottomset reflections in seismic sections. The Torok Formation is mostly silty mudstone that repre sents a spectrum of facies ranging from the toe of a marine slope to the outer shelf. This formation includes sandstone beds that are interpreted as deposits of basin-floor sub marine fans and incised slope channels (Molenaar, 1985, 1988; Bird & Molenaar, 1992; Houseknecht & Schenk, No claim to original US government works 645 Journal Compilation Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists D. W. Houseknecht et al. Ma SW NE Known Oil Source Rocks Study Interval 1 Nanuk oil pool 2 Qannik oil pool Nonmarine elastics Marine shelf elastics Marine basin clastics . Condensed marine shale -- - Marine carbonates ..-.---i Metasedimentary Granite I Hiatus or erosion Fig. 2. Stratigraphy in National Petroleum Reserve in Alaska (NPRA) showing tectonostratigraphic sequences (at left) and stratigraphic positions of study interval, known oil source rocks and Nanuk and Qannik oil pools. LCU, Lower Cretaceous unconformity. Modified from Houseknecht & Bird (2004). 2001, 2007). TheTorok Formation is expressed mostly as foresets in seismic sections. The Nanushuk Formation in cludes mudstone, sandstone and coal beds that similarly represent a spectrum of facies, including deposits of the marine shelf, shoreface, deltas, fluvial systems and coastal plain interfluves (Ahlbrandt et al., 1979; Huffman etal., 1985, 1988; LePain & Kirkham, 2001; LePain et al., 2008). The Nanushuk Formation is expressed as topsets in seismic sections. In the study area, clinoforms in Lower Cretaceous strata are part of a first-order depositional sequence that extends across the entire width of Arctic Alaska (Fig. 1). The wes tern part of this sequence onlaps the Chukchi platform and displays proximal seismic facies that do not include clinoforms. To the east, Lower Cretaceous clinoforms are present from the central Chukchi Sea to an ultimate (farthest basinward) shelf margin (illustrated with seismic data by Houseknecht & Schenk, 2005) that defines the eastern limit of the study area (Figs 1 and 3). Thus, the clinoform sequences in the study area were primarily fed by a longitudinal sediment dispersal system that was 300 500 km long (from vicinity of Herald arch and Chukchi platform; Fig. 1) and may have been influenced by a trans verse sediment dispersal system that was >75 km long (from ancestral Brooks Range; Fig. 1). The distal part of the sequence, east of the ultimate shelf margin, includes a wedge of mudstone and sandstone representing slope facies that grade basinward into condensed mudstone (Fig. 1). Palaeocurrent indicators in the clinoform sequence reflect the complexity of regional sediment dispersal patterns. Foreset dip observed in two-dimensional (2D) seismic data (this study; Bird & Andrews, 1979) indicates a predo minance of longitudinal eastward progradation within the foreland basin (Fig. 1). Exceptions occur in the following two areas where northeastward dip of clinoforms is evi dent: (1) in the north, where the depositional system over stepped the rift shoulder, and (2) in the south, where progradation into the foredeep from the ancestral Brooks Range is inferred (Fig. 1). Palaeocurrent data from the topset facies (Nanushuk Formation) are limited to outcrops in the Brooks Range foothills. These data indicate mostly northward sediment dispersal, except in the west where an eastward component of sediment dispersal is indicated (Fig. 1). METHODS Regional analysis ofdepositional sequences was completed using a grid of public-domain, mostly 1974-1981 vintage, 2D seismic data (Fig. 3; Miller et al., 2000). This grid was supplemented by proprietary, 1980-1995 vintage, 2D seis mic data along the northern and eastern margins of the study area (Fig. 3). Sparse well control (Fig. 3) was inte grated into the analysis by correlating wireline logs to seis mic data using synthetic seismograms generated from sonic logs. Depositional facies and sequences were interpreted by integrating seismic expression, wireline log patterns, core descriptions and biostratigraphic data as described else where (Houseknecht & Schenk, 2001). Results offield work in the Brooks Range foothills also were integrated into these interpretations (Houseknecht & Schenk, 2001, 2007; Houseknecht et al., 2007), although it generally is not possible to correlate depositional sequences defined in seismic data directly to specific outcrops because of structural complexity in the Brooks Range foothills and poor near-surface seismic resolution. Neither the lateral continuity of outcrops nor biostratigraphic control are adequate for correlating and dating specific bounding sur faces and stratigraphic sequences. No claim to original US government works 646 Journal Compilation Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Seismic analysis of clinoform depositional sequences, Alaska Fig. 3. Map of study area showing locations of several lowstand shelf margins, the Fish Creek slide (Homza, 2004), Nanuk (N) and Qannik (Q) oil pools, seismic images shown in Figs 4 and 5, seismic lines used in this study (greydotted lines), and exploration wells used in this study (white dots with black outlines). Structural deformation precludes correlation of sequence boundaries and shelf margins south of dotted line labelled `structural limit of mapping'. CLINOFORM SEQUENCES Analysis of Lower Cretaceous clinoform sequences is based on a regional framework established by mapping of high-amplitude seismic reflections that correlate to beds of condensed mudstone in wells. Hence, the mapping of our depositional sequences is related to the genetic-se quence stratigraphic approach of Galloway (1989). These beds of condensed mudstone, which are interpreted as flooding events, can be recognized even in low-resolution seismic data, and they define a repetitive motif of stratal geometry that is the basis for our regional mapping. This generalized motif is inferred to represent four stages of clinoform sequence development, including lowstand, transgression, highstand-aggradation and highstandprogradation (Fig. 4). Lowstand The base of each clinoform sequence is defined by abrupt basinward termination of topset strata at the shelf edge and local termination of foreset strata on the upper to middle slope (Fig. 4). These terminations are inferred to indicate the presence of an erosional surface (unconfor mity) on the outer shelf and upper slope. Above the uncon formity, most clinoform sequences display a thin and seismically transparent interval that onlaps at mid-slope and generally thickens basinward as it offlaps and becomes interbedded with high-amplitude reflections that coalesce with the GRZ (Fig. 4). Well penetrations show that this transparent interval is mostly sandstone, and interpreta tions of cores and outcrops suggest that this sandstone- rich interval includes deposits of sediment-gravity flows and turbidites (Houseknecht & Schenk, 2001, 2007). Higher up the slope and above the unconformity, some clinoform sequences include a wedge of strata that onlaps the outermost shelf or uppermost slope and downlaps the middle slope (Fig. 4). Most clinoform sequences, however, contain no such wedge of strata on the upper slope (Fig. 4). The unconformity at the base of each clinoform se quence is interpreted as a sequence boundary, and the im mediately overlying strata are interpreted as a lowstandsystems tract (LST).The seismically transparent intervals that onlap the mid-slope and interfinger basinward with the GRZ are interpreted as the deposits of submarine-fan aprons and basin-floor fan systems (Houseknecht & Schenk, 2007). At mid to lower slope, where it onlaps the sequence boundary, the LST likely includes incised sedi ment-gravity-flow deposits (slope channels), as inferred from seismic reflection geometries and studies of Torok successions in outcrop and core (Houseknecht & Schenk, 2001,2007).Wedges of strataperched on the upper slope of some clinoform sequences are interpreted as the deposits of lowstand shelf-margin deltas.Thus, the basal part of the clinoform motif is interpreted as the product of a late falling stage through lowstand in relative sea level (Fig. 4). It is likely equivalent to the LSTs interpreted by McMillen (1991). Transgression The inferred LST is overlain by a thin interval of strata, characterized by high-amplitude seismic reflections, that downlaps basinward and coalesces with the GRZ (Fig. 4). No claim to original US government works 647 Journal Compilation Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists D. W. Houseknecht et al. P> Flatten Datum <CJ --------SM Trajectory Path - - - - Sequence Boundary GRZ Faults (mass failure) Ka) Kb) __ __ _ Fig. 4. Non-interpreted (a) and interpreted (b) images of public-domain seismic data illustrating clinoform geometry and generalized interpretations of clinoform-sequence stratigraphy. Images are attened on a prominent topset seismic reection. Location of Husky Inigok #1 well is shown in (a), and g-ray log of the clinoform succession is shown in (b). Inset curve of relative sea level (RSL) with no accommodation included illustrates generalized interpretation of seismic expression inferred to represent lowstand (L), transgression (T), highstand-aggregation (A) and highstand-progradation (P). Positive reection coefficient represented by black in colour display and by white in greyscale display. Negative reection coefficient represented by red in colour display and by black in greyscale display. Images are plotted in time and approximate depth scale based on average velocity of clinoform sequence also is shown. Large yellow triangles indicate approximate dip. See text for additional explanation. Location of line segment is shown in Fig. 3. Up dip, this interval typically onlaps the sequence bound ary (or the overlying lowstand-shelf-margin-delta depos its, where present) on the middle to upper slope. In most clinoform sequences examined in this study, this interval rolls over into high-amplitude topset reections on the outer shelf. In other clinoform sequences, this interval is absent on the middle to upper slope and on the lower slope displays contorted or duplex repetitions (Fig. 4). In those cases, high-amplitude topset reections terminate abruptly at the shelf margin. The high-amplitude topset reections typically grade into medium- and then lowamplitude reections over a distance of 30-50 km land ward of the shelf margin. This interval is interpreted as a transgressive systems tract (TST) comprising condensed mudstone that accu mulated when marine waters drowned the shelf during ris ing sea level (Fig. 4). The high-amplitude reections correlate to lower velocity shale, likely higher in organic content than other slope facies, and are essentially basinward-coalescing tongues of the GRZ (Creaney & Passey, 1993). The deformation commonly displayed by this trans gressive facies and overlying deposits suggests that con densed mudstones of the TST commonly served as the sole for slope failure. Highstand aggradation The TST is overlain by an interval characterized by thick topset strata that display evidence of significant vertical aggradation and little basinward progradation of the shelf margin. Seismic geometries commonly display 300-500 m of vertical aggradation of the shelf margin with just a few kilometres of progradation (Fig. 4).The shelf margin is de fined by topset seismic facies with moderate- to high-am plitude reections that terminate or offlap into slope facies. In some clinoform sequences, the topset reections display basinward-thickening, wedge-shaped geometries near the shelf margin. Basinward from the aggradational shelf margin, correlative seismic facies display a poorly de fined foreset geometry comprising low- to medium-am plitude reections that commonly display contorted geometries. At the toe of the slope, broadly mounded, contorted and duplex repetitions of reections are com monly present within this seismic interval. No claim to original US government works 648 Journal Compilation Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association ofSedimentologists Seismic analysis of clinoform depositional sequences, Alaska This interval is interpreted as the aggradational part of a highstand systems tract comprising marine shelf and slope sediments that accumulated during late rising stage to highstand of relative sea level (Fig. 4) and near equilibrium between sediment-inux and accommodation.The stratal geometry suggests that a large volume of sediment accu mulated on the broad expanse of the shelf and a relatively modest volume accumulated on the smaller area ofthe slope prism, where facies are predominantly silty mudstones that display seismic evidence of mass wasting at small to med ium scale. Mounded, contorted, and duplexed seismic fa cies at the toe of slope are interpreted as piles of sediment displaced basinward by slumping or sliding. Highstand progradation Highstand-aggradational facies are overlain by an interval characterized by thick foreset strata and thin topset strata that display evidence of significant progradation and little aggradation of the shelf margin. Seismic geometries com monly display 20 km or more of shelf margin progradation with just 100-200 m of topset aggradation (Fig. 4). The topset strata are defined by moderate- to high-amplitude seismic reections that step basinward across the top of a large volume of crudely defined foreset strata. Internally, these foreset strata display low- to medium-amplitude seismic reections that toplap the overlying beds and display contorted geometries. The foreset strata typically display downlap onto theTST (Fig. 4). This interval is interpreted as the progradational part of a highstand systems tract comprising marine shelf and slope sediments that accumulated during late highstand to early falling stage of relative sea level (Fig. 4), when sedi ment inux exceeded accommodation. The stratal geome try of this interval suggests that a relatively large volume of sediment accumulated on the slope as compared with the shelf. Slope facies are mostly silty mudstone and display seismic evidence of mass wasting at medium to large scale. The large volume of mud that accumulated on the slope appears to have been highly prone to slope failure, ranging from relatively ductile creep and slumping that produced large masses of coherent seismic reections displaying folded and duplexed internal geometries to relatively uidized failure that produced mounds of seismic reections with chaotic internal geometries. trajectories that commonly occur in Albian clinoform se quences are discussed below (Fig. 5). Case 1: negative shelf-margin trajectory across sequence boundary In some clinoform sequences, the toplap surface steps downward in a basinward direction across the sequence boundary (Fig. 5a and b). In this case, the oldest topset seismic reections that onlap the sequence boundary are lower in elevation than the toplap surface below the se quence boundary. This geometry suggests that relative sea level during erosion of the sequence boundary fell be low the shelfedge, and perhaps below the toplap surface of the preceding HST (Fig. 5b). In this case, the LST includes a wedge of shelf-margin strata perched high on the slope just basinward of the knickpoint.This wedge of strata likely represents a lowstand, shelf-margin delta (Fig. 5b). Note that the inferred LST is actually a composite of an older wedge of sediment that completely onlaps the sequence boundary on the upper slope and a younger wedge of sedi ment that onlaps the sequence boundary a short distance landward of the knickpoint. Within the older lowstand wedge, the toplap surface steps basinward in both aggradational and progradational steps, although the former is predominant. In contrast, within the younger lowstand wedge, the toplap surface is strongly progradational. Alter natively, the younger wedge of sediment may represent a smaller scale sequence. Each lowstand wedge is capped by a high-amplitude seismic reection, which likely represents a condensed mudstone that accumulated during ooding. The younger high-amplitude seismic reection is slumped off the upper slope and is present near the base of the slope as a slide complex (not shown in Fig. 5b). The high-amplitude seismic reections that cap the topset strata of the two lowstand wedges coalesce landward near the knickpoint. We interpret the younger high-amplitude seismic reection as the approximate maximum ooding surface (MFS). Above the MFS, this clinoform sequence displays a small amount of shelf-margin aggradation followed by sig nificant progradation in the HST (Fig. 5b). The slight ne gative trajectory of the toplap surface in the HST is interpreted to result from compaction of the thick foreset mud during progradation. SHELF-MARGIN TRAJECTORIES In this section, we use higher resolution 2D seismic data to refine our interpretations of shelf-margin trajectories and clinoform-sequence development.We generally follow the concepts, if not the specific terminologies, of Galloway (1989), Helland-Hansen & Martinsen (1996), Steel & Olsen (2002), Porebski & Steel (2003), Bullimore etal. (2005) and Johannessen & Steel (2005). Throughout this discussion, toplap surfaces or offlap slope breaks are used to define shelf margin trajectories. Three distinct shelf-margin Case 2: positive shelf-margin trajectory across sequence boundary In other clinoform sequences, the toplap surface steps up ward in a basinward direction (apparent aggradation in seismic data) across the sequence boundary (Fig. 5c and d). This geometry suggests that relative sea level during erosion of the sequence boundary fell to a level no lower than the shelf edge of the preceding HST, and was higher than the toplap surface of the preceding HST (Fig. 5d). In this case, the LST is a wedge of sediment perched mostly on the slope, and the topset seismic reections of the LST No claim to original US government works 649 Journal Compilation Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Seismic analysis of clinoform depositional sequences, Alaska DISCUSSION Systems tract interpretations Interpretations of sequence stratigraphy presented in the preceding sections include uncertainty, especially in the assignment of strata to systems tracts. Our interpretations are based largely on the recognition in seismic data of (1) reection terminations at the shelf margin and on the upper slope and (2) high-amplitude reections that either onlap the middle to upper slope or roll over into topset reections at the shelf margin, and that downlap basinward and coalesce with the GRZ (Figs 4 and 5). These features are interpreted as (1) sequence boundaries and (2) a drape of transgressive mudstone, respectively. We have interpreted all strata above the inferred se quence boundary and below the high-amplitude drape as a LST. However, we cannot preclude the possibility that part - or all - of these strata were deposited during rising stage and might more appropriately be interpreted as part of theTST Similarly, we have interpreted all strata above the high-amplitude drape and below the next higher se quence boundary as a HST, which we subdivide into aggradational and progradational segments based on shelfmargin trajectories. However, both MFS (contact between TST and HST-aggradation) and the successive sequence boundary are difficult to pick precisely in 2D seismic data. Although our interpretations are based on all available information, including wireline log and core interpreta tions (Houseknecht & Schenk, 2001), insufficient well control is available to constrain the interpretation of every shelf margin considered in this study (Fig. 3). Our inter pretation of systems tracts based on well-defined seismic criteria, therefore, is probably the most objective approach for regional mapping. Reservoir implications In all three examples of shelf-margin trajectories, two sig nificantly progradational HST shelf-margin trajectories are separated by net aggradation across a relatively short distance. However, the `net aggradation' involves very dif ferent shelf-margin stratal geometries and depositional processes. In case 1, net aggradation includes a negative step across a sequence boundary, multiple progradation and aggradation steps within a LST, and aggradation during and following maximum ooding (Fig. 5b). In case 2, net aggradation includes a positive step across the sequence boundary, progradation and minor aggradation within the LST, and aggradation during and following maximum ooding (Fig. 5d). In case 3, net aggradation includes mass-failure removal of strata deposited during a net rise in relative sea level and a significant backstepping of the toplap surface. The potential for coarse sediment by-pass and sand stone reservoir quality are different in these three cases. Case 1, which represents the most significant drop in rela tive sea level, holds the greatest potential for coarser- grained reservoir facies in lowstand deposits, both in shelf-margin deltas and in lower slope channel and fan systems. The recognition of a lowstand shelf-margin wedge onlapping the upper slope is an indication that se diment was delivered to the shelf margin and beyond.This stratal geometry also favours the occurrence of relatively coarse grained, incised channel deposits in uvial systems landward of the shelf margin. Case 2, which represents a smaller drop in relative sea level, is probably the most com monly observed in Albian clinoform sequences in NPRA and is less likely to be characterized by significant by-pass of coarser sediment. In this case, a shelf-margin sediment wedge onlapping the outer shelf may have been prone to extensive wave reworking of generally fine-grained sand, a combination demonstrated to produce generally poor re servoir quality in this basin (LePain eta/., 2008). Moreover, in this case, there is less potential for transport of coarsergrained sand to deeper-water depositional systems. Case 3, which appears to represent a mass-failure process, carries no implications regarding the presence or quality of sand stone reservoirs. Shelf-margin orientation and clinoform dimensions The criteria described above were used to map lowstand shelf margins in the study area (Fig. 3). Across the central part of the area, which represents the gradation between the southern ank of the Beaufort rift shoulder and the northern ank of the Colville foreland basin, the overall shelf-margin orientation swings from ^330 in the west to ^0 in the east (Fig. 3). To the north, all the shelf mar gins turn westward to approximately parallel the northern margin of the rift shoulder (Figs 1 and 3), across which ac commodation increased abruptly into the Canada basin. Along the southern margin ofthe area, there are subtle in dications that the shelf margins turn eastward to parallel the axis of the foredeep, and this is confirmed by regional mapping of the ultimate slope wedge east of the study area and by the presence of outcrops of Albian topset strata in the Brooks Range foothills east of the study area (Figs 1 and 3). This eastward turn of lowstand shelf margins likely reects the inux of sediment into the foredeep from the an cestral Brooks Range to the south, as suggested by palaeocurrent data from topset strata (Fig. 1). Throughout the study area, the foreset-dip direction of these clinoform sequences is consistently perpendicular to the lowstand shelf margins. The dimensions of clinoform sequences generally reect their location relative to the Beaufort rift shoulder and Colville foredeep. In the northern part of the study area, where the clinoform sequences overstep the relatively high-standing rift shoulder, total clinoform sequence thickness is 600-1000 m.This dimension increases gradu ally southward and is 1700-2500 m at the structural limit of mapping (Fig. 3), located on the north ank of the fore deep. This southward increase in clinoform-sequence No claim to original US government works 651 Journal Compilation Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists D. W. Houseknecht et al. thickness is a direct indication of increased accommodation from the rift shoulder into the foredeep. Moreover, this range of thickness provides an approximate indication of water depth during deposition. The foreset length (dis tance from toplap to downlap ofa seismic reection) varies from an average of ^30 km (25-40 km range) in the north to ^70 km (45-125 km) in the south. Average foreset dip, calculated as a linear slope from toplap to downlap, ranges from 0.6 to 2.5. No spatial variation in foreset dip is evi dent across the accommodation domains, although both the sequence boundaries and depositional foresets are asymptotic (Fig. 4). On the upper slope, maximum dips are estimated to be 3-8 on both erosional sequence boundaries and depositional foresets. From the lower slope basinward, maximum dip is typically < 1.The stee pest dip associated with the clinoform sequences is ob served on the extensional decollements associated with mass failure on the upper slope (Fig. 5f). These decollement surfaces commonly display dips > 10 and locally as much as 20, although these dips are difficult to estimate from the available seismic data. The Albian clinoform sequences described in this pa per are significantly larger than most reported in the lit erature. For example, well-documented clinoform sequences in the Central Tertiary basin of Spitsbergen (Johannessen & Steel, 2005), Porcupine basin of offshore Ireland (Johannessen & Steel, 2005), Karoo basin of South Africa (Johnson etal., 2001; Wild etal., 2007),West Siberian basin of Russia (Zharkov, 2001; Ulmishek, 2003) and Car pathian foredeep of Poland (Porebski etal., 2003) typically display a thickness of ^250-500 m. Most examples of thicker clinoforms occur on passive margins and, even in that setting, a thickness > 1 km is rare (Adams & Schlager, 2000; Porebski & Steel, 2003; Cummings & Arnott, 2005). Such unusually thick clinoform sequences are interpreted as having formed as the result of significant subsidence followed by rapid sediment inux at the temporal scale of the Lower Cretaceous first-order depositional sequence, although this generalized interpretation may be revised as an ongoing analysis of Cretaceous strata in Arctic Alaska is completed.The range of foreset dip in the Albian clinoforms is similar to other examples cited, except for the Karoo basin, where dip is typically < 0.6 (Johnson et al., 2001; Wild etal, 2007). In the Alaska North Slope, Lower Cretaceous (Albian) clinoform sequences display the following three common shelf-margin trajectories that appear aggradational in low-resolution seismic data: (1) A negative step across the sequence boundary followed by aggradation >progradation in the LSTand mostly progradation in the HST. This geometry occurs when relative sea level falls below the to plap surface of the preceding HST (2) A positive step across the sequence boundary followed by progradatio n> aggradation in the LST and mostly progradation in the HST. This geometry occurs when relative sea level falls to a level not lower than the shelf edge of the preceding HST. (3) A retrogradational backstep occurs when a mass-failure decollement develops along or near a se quence boundary during highstand. In all these cases, the shelf-margin trajectory is predominately progradational in the HST. Recognition of these contrasting shelf-margin geometries may provide insights for anticipating the pre sence and quality of sandstone reservoirs in both deepand shallow-water facies of Albian clinoform sequences. Lowstand shelf-margin orientations, which are consis tently perpendicular to clinoform foreset dip direction, reect the longitudinal west-to-east filling of the foreland basin across most of the study area. In the north, where the clinoform depositional systems overstepped the Beau fort rift shoulder, lowstand shelf margins turn westward and approximately parallel the northern margin of the Beaufort rift shoulder, which represents a huge accommo dation increase into the Canada basin to the north. In the south, the lowstand shelf margins appear to turn eastward, likely reecting inux of sediment into the foredeep from the ancestral Brooks Range. The Albian clinoform sequences of the Alaska North Slope are significantly thicker than most reported in the literature, suggesting significant subsidence followed by rapid sediment inux. But how this generalized interpre tation fits into the regional tectonic history awaits further analysis. ACKNOWLEDGEMENTS The manuscript was improved by constructive reviews by Paul Decker, Ashton Embry, Sverre Henriksen, Chris Swezey, Jim Coleman and two anonymous reviewers. CONCLUSIONS Low-resolution seismic data can be used to identify and map clinoform depositional sequences by using stratal geometry, foreset-dip directions, and interpretations of shelf-margin trajectory. Sequence boundaries and lowstand shelf margins can be inferred near the base of seis mic intervals with apparently aggradational shelf-margin trajectories. However, a higher resolution approach is re quired to identify the components of aggradational trajec tories and to interpret the character ofassociated sequence boundaries and lowstand deposits. REFERENCES Adams, E.W. & Schlager W. (2000) Basic types of submarine slope curvature. J Sed. Res., 70,814--828. Ahlbrandt, T.S., Huffman, A.C., Fox, J.E. & Pasternack, I. (1979) Depositional framework and reservoir quality studies of selected Nanushuk Group outcrops, North Slope, Alaska. In: Preliminary Geologic, Petrologic, and Paleontologic Results of the Study ofNanushuk Group Rocks, North Slope, Alaska (Ed. by TS. Ahlbrandt), US Geol. 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Manuscript received 11 April 2008; Manuscript accepted 29 November 2008 No claim to original US government works 654 Journal Compilation Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists North Chukchi Basin / 47/ r\ \ yW j X \/ \ // . / \ \ <T D \PBJ Chukchi Sea \ NPRA Canada Basin Clinothem Paleocurrents Seismic Facies Foreset Dip I____| Proximal Topset X-beds Clinoform | Slope wedge Beaufort Sea _| Distal condensed ^Terminal LST ------FDW pinchout Fish Creek slide 1002 Area ANWR