The Ulungarat Basin of Arctic Alaska is a unique exposed stratigraphic record of the mid-Paleozoic transition from the Romanzof orogeny to post-orogenic rifting and Ellesmerian passive margin subsidence. The Ulungarat Basin succession is composed of both syn-rift and post-rift deposits recording this mid-Paleozoic transition. The syn-rift deposits unconformably overlie highly deformed Romanzof orogenic basement on the mid-Paleozoic regional angular unconformity and are unconformably overlain by post-rift Endicott Group deposits of the Ellesmerian passive margin. Shallow marine strata of Eifelian age at the base of the Ulungarat Formation record onset of rifting and limit age of the Romanzof orogeny to late Early Devonian. Abrupt thickness and facies changes within the Ulungarat Formation and disconformably overlying syn-rift Mangaqtaaq Formation suggest active normal faulting during deposition. The Mangaqtaaq Formation records lacustrine deposition in a restricted down-faulted structural low. The unconformity between syn-rift deposits and overlying post-rift Endicott Group is interpreted to be the result of sediment bypass during deposition of the outboard allochthonous Endicott Group. Within Ulungarat Basin, transgressive post-rift Lower Mississippian Kekiktuk Conglomerate and Kayak Shale (Endicott Group) are older and thicker than equivalents to the north. North of Ulungarat Basin, deformed pre-Middle Devonian rocks were exposed to erosion at the mid-Paleozoic regional unconformity for ∼50 m.y., supplying sediments to the rift basin and broader Arctic Alaska rifted margin beyond. Although Middle Devonian to Lower Mississippian chert- and quartz-pebble conglomerates and sandstones across Arctic Alaska share a common provenance from the eroding ancestral Romanzof highlands, they were deposited in different tectonic settings.

In Arctic Alaska, the Devonian is a period of profound tectonic change encompassing assembly of accreted basement during the Romanzof orogeny, development of a new paleo-Pacific rift margin, and initiation of passive margin sedimentation across the resulting platform (e.g., Churkin et al., 1985; Hubbard et al., 1987; Moore et al., 1994; Lane, 2007). The stratigraphic record of this tectonic transition is, however, missing or obscured due to widespread mid-Paleozoic erosion, as well as Mesozoic burial, deformation, and metamorphism during the Brooks Range orogeny.

In order to examine the mid-Paleozoic framework of Arctic Alaska, it is first necessary to understand the overprint of Mesozoic and Cenozoic Brooks Range fold-thrust belt deformation. In Jurassic to Early Cretaceous time, the contractional Brooks Range fold-thrust belt detached Upper Devonian and younger passive margin cover of the Arctic Alaska margin from its basement, shortening it into a stack of far-traveled, thin-skinned allochthons now widely exposed in the modern Brooks Range Mountains (Fig. 1; Moore et al., 1994). The pre-Middle Devonian orogenic basement that underlies the mid-Paleozoic regional unconformity beneath the North Slope and northeast Brooks Range represented the autochthon to Mesozoic Brooks Range deformation (Figs. 1 and 2; Moore et al., 1994). Present-day topography is the result of additional basement-involved shortening that occurred in Cenozoic time, uplifting the central and eastern Brooks Range and forming a parautochthonous salient in the northeastern Brooks Range (Fig. 1; Moore et al., 1994).

The Ulungarat Basin (new name), exposed in the parautochthonous northeastern Brooks Range (Figs. 1 and 2), contains a unique stratigraphic record of the Middle Devonian to Early Mississippian tectonic transition, minimally affected by later deformation (Anderson et al., 1994). The Ulungarat Basin succession (Anderson, 1993; Anderson et al., 1994) dates the end of Romanzof deformation, constrains initiation and end of Devonian rifting, resolves the nature and age of mid-Paleozoic unconformities, and provides a stratigraphic link between Devonian and Mississippian allochthonous and autochthonous passive margin successions across Arctic Alaska.

In this paper we document the stratigraphy of the Ulungarat Basin and establish its relationship to Devonian and Early Mississippian regional tectonic events. The Ulungarat Basin succession is composed (in ascending order) of the (1) Ulungarat Formation, (2) Mangaqtaaq Formation, (3) Kekiktuk Conglomerate, and (4) Kayak Shale (Reiser et al., 1980; Anderson et al., 1994). The Ulungarat Formation and Mangaqtaaq Formation are here elevated to formal status by definition of type sections in the northeastern Brooks Range along with detailed lithologic descriptions, fossil age control, and regional variations. The overlying Kekiktuk Conglomerate and Kayak Shale of the Ulungarat Basin belong to the basal Endicott Group of the Ellesmerian passive margin succession (Fig. 2). The character of tectonic unconformities at the base of the Ulungarat Formation and between the Mangaqtaaq Formation and Kekiktuk Conglomerate are examined to establish their nature and origin. We make the case that the Ulungarat and Mangaqtaaq formations constitute a relatively complete, preserved syn-rift sequence of the Arctic Alaska margin.

The Middle Devonian to Lower Mississippian Ulungarat Basin is part of the post-orogenic cover sequence of Arctic Alaska, a large tectonostratigraphic terrane that underlies most of Alaska north of the Arctic Circle and extends into northwestern-most Canada (Fig. 1; Jones et al., 1986, 1987; Moore et al., 1994). The Devonian to Mississippian rocks of Arctic Alaska record a time of profound tectonostratigraphic change resulting from continental collision, rifting, and passive margin subsidence (Churkin, 1975; Churkin et al., 1985; Moore et al., 1994).

Late Early Devonian Romanzof Orogeny

The Romanzof orogeny (Lane, 2007) resulted in the tectonic assembly and deformation of terranes of Greater Baltican, Siberian, Laurentian, and Iapetan affinity during the final phase of mid-Paleozoic closure of an intervening oceanic basin (Colpron and Nelson, 2011; Strauss et al., 2017). Deformed deep oceanic rocks, exposed in the Romanzof Mountains of the parautochthonous northeastern Brooks Range (basinal succession and Whale Mountain allochthon; Johnson et al., 2016, 2019; Strauss et al., 2019) are interpreted as remnants of a subducted early Paleozoic Iapetan ocean basin that once separated Siberia and Laurentia (Colpron and Nelson, 2009; Lawver et al., 2011).

In most of the North Slope and northeastern Brooks Range, Romanzof deformation is capped by a mid-Paleozoic regional angular unconformity above which the oldest overlying strata are the Lower to Middle Mississippian Kekiktuk Conglomerate of latest Tournaisian to Visean age (basal Ellesmerian megasequence, Fig. 2; Tailleur et al., 1967). In the southern Romanzof Mountains, basal strata of the Ulungarat Formation of Eifelian age (early Middle Devonian) rest in angular unconformity above folded strata of the Romanzof formation (Reiser et al., 1980; Romanzof chert of Mull and Anderson, 1991; Johnson et al., 2019).

Middle to Late Devonian Rifting

Rocks below the mid-Paleozoic regional unconformity are locally intruded by peraluminous Devonian upper-crustal granites, which postdate deformation but predate the unconformity (Nelson and Grybeck, 1980; 375–368 Ma, Lane and Mortensen, 2019; Ward et al., 2019). This intrusive episode began with mafic magmatic underplating and partial melting of lower crust in early Middle Devonian, triggering crustal extension, followed by Late Devonian emplacement of plutons at shallow crustal levels (Lane and Mortensen, 2019).

Post-orogenic extension is well documented in the subsurface of the North Slope and Chukchi Sea (Grantz and May, 1988; Kirschner and Rycerski, 1988; Sherwood et al., 2002), where seismic-reflection lines reveal numerous subsurface normal faults interpreted as active from mid-Devonian to early Mississippian time (Fulk, 2010; Kumar et al., 2011). Gravity, magnetic, and seismic-reflection data reveal a deep east-west– to northwest-trending rift basin system beneath the southern North Slope and Chukchi Sea (Fig. 1; Utikok Basin of Kelley, 1999; Hanna trough of Sherwood et al., 2002). A large, long-wavelength magnetic anomaly is interpreted as the expression of crustal thinning and rift-related mafic intrusions (Bassinger, 1968; Saltus et al., 2001, 2002). Seismic-reflection lines in the North Slope subsurface also image smaller northwest-trending post-orogenic mid-Paleozoic extensional basins, including the Ikpikpuk-Umiat and Meade basins (Fig. 1; Grantz and May, 1988; Mauch, 1989; Grantz et al., 1990), which contain up to 3000 m of fault-bounded clastic graben fill of probable Late Devonian or Early Mississippian age (Kirschner and Rycerski, 1988; Moore et al., 1994; Fulk, 2010). Two wells, South Meade #1 and Topagoruk #1, penetrated this deep section (Dumoulin, 2001), but fossils were only recovered from unmetamorphosed clastic rocks in core from the Topagoruk #1 well (Fig. 1), where Collins (1958) reported interbedded plant fragments of Early(?) to Middle Devonian age.

Additional evidence for Middle Devonian rifting is found in the Ambler Group (Ambler District; Fig. 1; Dillon et al., 1980; Hitzman et al., 1986; Hoiland, 2019), Beaucoup Formation (Fig. 1; Dutro et al., 1979; Dillon, 1989; Dumoulin and Harris, 1994), and Angayucham terrane (Fig. 1; Pallister et al., 1989) of the south-central Brooks Range. The Ambler volcanic and volcaniclastic suite is the extrusive equivalent of Devonian upper crustal felsic plutons (Dillon and Tilton, 1985), which postdate deformation of rocks below the mid-Paleozoic regional unconformity. The Beaucoup Formation is a stratigraphically complex unit, with variable thickness and internal unconformities, composed of variably metamorphosed marine clastics, carbonates, silicic volcaniclastics, and local mafic intrusions (Dillon et al., 1987a; Dumoulin and Harris, 1994; Moore et al., 1994). Based on conodonts, corals, stromatoporoids, brachiopods, and mollusks, it has been dated as latest Middle Devonian to early Late Devonian (Brosgé and Reiser, 1964; Brosgé et al., 1979; Dillon et al., 1988; Dumoulin and Harris, 1994). Marine carbonates associated with pillow basalts in basal imbricate thrust sheets of the Angayucham oceanic terrane, interpreted as originally deposited at the edge of the rifted continental margin, are dated as Middle Devonian to Early Mississippian based on conodonts, corals, crinoids, brachiopods, and cephalopods (Pallister et al., 1989).

Late Devonian to Mississippian Passive Margin Subsidence

The Ellesmerian megasequence of Arctic Alaska (Lerand, 1973; Grantz et al., 1981; Hubbard et al., 1987) is the depositional record of a mid-Paleozoic to mid-Jurassic south-facing (present coordinates) passive continental margin (Fig. 2; Dutro, 1981; Moore et al., 1992, 1994). The basal clastic succession of the Ellesmerian passive margin is the Upper Devonian to Middle Mississippian Endicott Group, which is widely exposed within the proximal Endicott Mountains allochthon and parautochthon of the northeastern Brooks Range (Figs. 1 and 2).

Two distinct stratigraphic successions are recognized within the Endicott Group, a thin (<∼100 m) autochthonous (and parautochthonous) succession to the north and a thick (>4000 m) allochthonous succession to the south, distinguished by differences in thickness, age range, and basal contact (Fig. 2; Mull et al., 1976; Nilsen, 1981). The allochthonous Endicott Group was originally deposited south of the autochthonous Endicott Group, and was displaced northward by Early Cretaceous thrusting of the Endicott Mountains allochthon during the Brooks Range orogeny (Mull, 1982; Mayfield et al., 1983; Oldow et al., 1987). The autochthonous and allochthonous Endicott successions are viewed as genetically related, but details of the original paleogeographic and depositional relationship between the two successions are not known due to the large displacement on the Endicott Mountains allochthon (Tailleur et al., 1967; Nilsen and Moore, 1984; Brosgé et al., 1988). The top of the Endicott Group in both successions is the transgressive Kayak Shale, which conformably underlies platform carbonate rocks of the Lisburne Group (Fig. 2; Brosgé et al., 1962; Tailleur et al., 1967).

The Upper Devonian to Lower Mississippian allochthonous succession of the Endicott Group, exposed in thrust sheets of the Endicott Mountains allochthon, is a progradational to retrogradational terrigenous clastic succession of >4000 m thickness (Figs. 1 and 2; Mull et al., 1976; Nilsen and Moore, 1984). Although commonly a thrust fault, the base of the allochthonous succession locally conformably overlies Beaucoup Formation of late Middle to early Late Devonian age (Dutro et al., 1979; Dumoulin and Harris, 1994). The Beaucoup Formation shows compositional affinity to the overlying Endicott Group (Dutro et al., 1979; Anderson, 1987; Dillon et al., 1987a).

The Lower to Middle Mississippian autochthonous succession of the Endicott Group is present in the northeastern Brooks Range, North Slope subsurface, Lisburne Peninsula, and Doonerak window (Fig. 1; Moore et al., 1984, 1994, 2002). The autochthonous succession is widely exposed in the northeastern Brooks Range, where Kekiktuk Conglomerate is a thin, discontinuous unit that unconformably overlies deformed basement with high-angle discordance (Brosgé et al., 1962; LePain et al., 1994). Here, Kekiktuk Conglomerate is of latest Tournaisian to earliest Visean age, is generally less than 100 m thick, and is overlain by transgressive marine Kayak Shale deposited in paleovalleys in a rift-flank region (Nilsen et al., 1980, 1981; LePain et al., 1994). On the North Slope, autochthonous Kekiktuk Formation of latest Tournaisian to earliest Visean age also generally unconformably overlies deformed early Paleozoic basement but locally overlies undeformed and unmetamorphosed clastic deposits in extensional basins of Devonian to Early Mississippian age (Mull, 1982; Grantz and May, 1988; Fulk, 2010). At Endicott Field (Fig. 1), Kekiktuk Formation is of latest Tournaisian to Visean age and displays thickness variability controlled by syn-depositional normal faults that offset the basement unconformity (Melvin, 1987; Woidneck et al., 1987; Ravn, 1991). The autochthonous succession is also exposed in Doonerak window (Fig. 1), a structural fenster in the Endicott Mountains allochthon (Brosgé and Reiser, 1971; Moore et al., 1994; Oldow and Avé Lallemant, 1998), where it lies unconformably on deformed pre-Middle Devonian basement and is gradationally overlain by Kayak Shale of middle Tournaisian age (Armstrong et al., 1976; Handschy, 1998; Hoiland et al., 2017). Stratigraphic and structural relationships support the interpretation that the Doonerak window exposes an uplifted (parautochthonous) older part of the autochthonous succession that lies below the allochthon (Dutro et al., 1976; Mull, 1982; Wissinger et al., 1998).

The mid-Paleozoic regional unconformity has been historically called the “pre-Mississippian” unconformity (e.g., Moore et al., 1994; Bird, 1999), due to Mississippian onlap and the inability to distinguish distinct Romanzof and Ellesmerian deformation events within the long unconformity gap (Lane, 2007). This may lead to the misconception that the age of basement deformation in Arctic Alaska is coeval with the Late Devonian to Early Mississippian Ellesmerian orogeny of Arctic Canada. The Ellesmerian orogeny in Arctic Canada deforms a conformable platform succession as young as Famennian, and is unconformably overlain by basal strata of the Sverdrup Basin of late Visean age (Thorsteinsson and Tozer, 1970; Trettin, 1991; Lane, 2007). The nature and extent of Ellesmerian deformation in Arctic Alaska is undocumented. Late Devonian to Early Mississippian folds and faults reported from Ikpikpuk-Umiat Basin (Fig. 1; Fulk, 2010), north side of Barrow arch (Houseknecht and Connors, 2016), and Endicott Basin (Fulk, 2010) have been ascribed to a transtensional tectonic regime, but the relationship, if any, of these structures to the Ellesmerian orogeny has not been established.

Geologic mapping was conducted by Anderson from spike camps in the southwestern Demarcation Point (A-4) quadrangle, totaling 20 weeks during the field seasons of 1988–1991. Field work defined stratigraphic units and documented structural relationships providing the basis for construction of a geologic map and structural cross sections. Traverses within the Romanzof formation included detailed structural measurement of two of the major chert lenses, to characterize deformation. Stratigraphic sections were described and measured in meters using a Jacob's staff, to record detailed observations of stratigraphic units and their lateral variations. Macrofossils were collected and sent out for identification (see Acknowledgments). Samples were also collected for conodont, palynology, and apatite fission-track analysis. Sandstones were collected from measured sections of the Ulungarat Formation for petrographic analysis. A minimum of 400 detrital grains were counted from each thin section using a 0.66 × 1.32 mm grid and a 10× objective. Counting methods, and criteria for determining mineral components and diagenetic alteration, follow Dickinson (1970, 1985), Dickinson and Rich (1972), Graham et al. (1976), and Dickinson et al. (1982).

The study area (Fig. 3; part of Demarcation Point A-4 quadrangle) includes the drainage divide between the headwaters of the Kongakut and Aichilik rivers. Little river (informal name), a tributary of the upper Kongakut River, lies on the southeast flank of a Cenozoic deep-seated, east-plunging regional anticlinorium cored by pre-Middle Devonian “basement” (see inset, Fig. 3). North-vergent Cenozoic thrust faults locally divide the overlying Middle Devonian to Mississippian stratigraphic succession into south-dipping thrust sheets (Fig. 3).

Two distinct stratigraphic successions are juxtaposed by the Aichilik Pass thrust (APT, Fig. 3). To the north, in the footwall, thin laterally discontinuous Lower Mississippian Kekiktuk Conglomerate (Mkt-1) overlies complexly deformed Ordovician chert and phyllite (Romanzof formation; Or) across a high-angle unconformity. This thin Mississippian stratigraphic succession together with the underlying deformed chert and phyllite are informally referred to as the West Fork Valley succession (WFVS, Fig. 4). The West Fork Valley succession is exposed in Little river valley and the headwaters of the Aichilik River (Fig. 5). To the south, in the hanging wall, the Middle Devonian to Mississippian clastic succession is much thicker and includes two formations that are not present to the north (Du and Dm, Fig. 3). This thicker succession is informally named the Continental Divide succession (CDS, Fig. 4). There are two thrust sheets within the Continental Divide succession, separated by the Kongakut River thrust (KRT, Fig. 3); these thrust sheets have minor differences in stratigraphic thickness and organization. To the west, along the upper headwaters of the Kongakut River, the Continental Divide succession is in direct depositional contact with deformed chert and phyllite of the Romanzof formation (Fig. 3, loc. A).

Stratigraphic relations indicate a system of basin-bounding normal faults was active during deposition of the Continental Divide succession (Fig. 4). Restoration of displacement on the Cenozoic Aichilik Pass thrust requires an original abrupt thickening of the stratigraphic section with addition of two new formations and thickening of the Kekiktuk Conglomerate. This change in stratigraphic thickness is best explained by a syn-depositional normal fault zone. However, no normal fault surface is observed, likely due to concealment beneath the Aichilik Pass thrust and within the highly deformed Romanzof formation.

Romanzof Formation

The Romanzof formation (informal name; Or, Figs. 3 and 5) consists of massive and bedded chert lenses in a matrix of 40%–60% phyllite (OCcp of Reiser et al., 1980). This structurally complex unit is part of the “Romanzof Mountains southern belt of pre-Mississippian rocks” described by Moore et al. (1992). The Romanzof formation is included in the Whale Mountain allochthon of Johnson et al. (2019) and Strauss et al. (2019), who interpret it to have been deposited in a deep ocean basin prior to being deformed during the Romanzof orogeny.

Romanzof formation is the stratigraphically and structurally lowest unit in the study area but forms topographic highs due to resistance of chert to erosion (Fig. 5A). The top of the assemblage is marked by a profound angular unconformity beneath overlying strata (Figs. 5B and 5C). Structural thickness of the unit is greater than 1000 m. Stratigraphic thickness is unknown, because the base is not exposed, and it is structurally thickened. In the field, massive to bedded cherts are black, various shades of gray, and white. Abundant radiolarian ghosts are visible in thin section. Chert lenses are deformed internally, displaying at least two generations of tight upright to isoclinal folds with variably plunging refolded axes. Chert lenses form mappable linear features, with groups of lenses extending for tens of kilometers in an east-west direction. Major chert lenses and folds and faults within them were rotated to steep dips prior to truncation by the overlying unconformity surface. In the study area, resistant chert knobs display as much as 15 m of relief on the unconformity surface.

Age

The Romanzof formation in the study area is of known Ordovician age but may include undated strata as young as Early Devonian and as old as Cambrian (Strauss et al., 2019). Reiser et al. (1980) assigned it an inferred Cambrian to Ordovician age. Moore and Churkin (1984) and Moore et al. (1992) recovered Ordovician graptolites from equivalent rocks along the Canning River. Strauss et al. (2019) reported a maximum depositional age of <ca. 460 Ma (late Middle Ordovician or younger) determined using U-Pb detrital zircon radiometric analysis on an interbedded sandstone. Radiolarian ghosts present in the chert are not well enough preserved to be age diagnostic. While locally the only definitive ages for this unit are Ordovician, Strauss et al. (2019) date the Whale Mountain allochthon as Cambrian to Early Devonian(?) based on regional work.

Ulungarat Formation

The Ulungarat Formation is a 395-m-thick, coarsening- and thickening-upward, terrigenous clastic succession that records a variety of shallow marine and non-marine basin margin depositional environments (Figs. 4 and 6; this study). Depositional contacts are well exposed in the western study area (Fig. 3, loc. A), where the Ulungarat Formation unconformably overlies deformed Romanzof formation. Farther east in the study area (Fig. 3), the base of the Ulungarat Formation is defined by the Aichilik Pass thrust (APT, Fig. 3), which cuts upsection, placing Ulungarat Formation in the hanging wall over Kekiktuk Conglomerate in the footwall (e.g., Fig. 3, north of loc. B). Thrust overlap and truncation of contacts indicate that displacement on the Aichilik Pass thrust is ∼3 km (Fig. 3). Detailed structural study shows that thrusting of the Ulungarat Formation occurred during the Cenozoic phase of the Brooks Range orogeny and that it was not affected by the complex polyphase contractional deformation of the underlying Romanzof formation (Anderson, 1993).

The succession is here formally named. The type section of the Ulungarat Formation (Fig. 3, loc. B; Fig. 6) is located at the east end of Ulungarat ridge (informal name) in the NW1/4 of Section 7, T5S, R38E, Demarcation Point A-4 quadrangle (69°14′N, 143°6.4′W). The study area is in a remote part of the Arctic National Wildlife Refuge where there is an absence of available geographic names for assignment to new formations. Therefore, new formations are named using words from the local Inupiat Eskimo language that describe the location. Ulungarat means “sloping ridge with a very steep side” (J. Nageak, University of Alaska Fairbanks, 1989, personal commun.), which describes the type locality. A detailed meter by meter description of the formation at the type locality can be found in Anderson (1991).

The upper contact of the Ulungarat Formation is an unconformity. In part of the area, the upper surface is disconformably overlain with low-angle discordance by the Mangaqtaaq Formation (Fig. 3, loc. B), while elsewhere, braided fluvial deposits of the Lower Mississippian Kekiktuk Conglomerate unconformably overlie the Ulungarat Formation (Fig. 3, locs. C, F, and E). On the basis of variations in lithology and internal organization, the Ulungarat Formation is divided into four informal members labeled, from base to top, A–D (Fig. 6). The formation consists of shallow marine deposits overlain by a coarsening and thickening upward fluvial succession. See Supplemental Material1 for a detailed description of the lithology and organization of the Ulungarat Formation.

Age

The Ulungarat Formation is of Middle to Late(?) Devonian age based on fossils (Fig. 6). Identification of marine invertebrate fossils in member A indicates an Eifelian (early Middle Devonian) age for the lower one-third of the formation. The green-gray mudstone at the base of the formation contains abundant large inarticulate brachiopods identified as Bicarinatina kongakutensis n. sp. of Eifelian age (Popov et al., 1994). In addition, the basal 18–27-m-thick mudstone contains Ladjia sp. (an ambocoelid brachiopod), as well as fragments of nuculoid bivalves, nautiloid cephalopods, and ramose bryozoans (R.B. Blodgett, 1992, written commun.). Interbedded sandstone deposits higher in member A contain Ulungaratoconcha heidelbergeri sp. nov. (a murchisonid gastropod) and Coelotrochium sp. (a dasycladacean alga) also dated as Eifelian (Blodgett and Cook, 2002; Blodgett et al., 2002; Blodgett, 2008). Additional invertebrate fauna recovered include several indeterminate species of bivalves (including nuculoid bivalves) and several species of brachiopods (including reticularid brachiopods). Species identified include Spinatrypa sp. and Naticopsis (Jedria) sp. cf. N. (J.) costatus D'Archiac and DeVerneuil (R.B. Blodgett, 1992, written commun.). One sample contained fish plates. Fauna recovered from the uppermost beds of member A include crinoids, dendroid tabulate corals, reticularid brachiopods, dechenellid trilobites, belleropohontid and straparollid gastropods, several species of bivalves (including pectenoid and nuculoid bivalves), and stick-like bryozoan (R.B. Blodgett, 1992, written commun.). Prior to collection as part of this study, the lingulid brachiopod, Bicarinatina kongakutensis and the murchisonid gastropod, Ulungaratoconcha heidelbergeri, had no previously reported occurrences from cratonic North America (Blodgett and Cook, 2002; Blodgett et al., 2002; Blodgett, 2008). They are reported only from occurrences in Baltica and the Russian platform. The dasycladacean alga, Coelotrochium, is also not known from cratonic North America. It is common in some of the accreted terranes (Farewell, Alexander, and Livengood; Blodgett and Cook, 2002; Blodgett et al., 2002; Blodgett, 2008).

The age of the upper Ulungarat Formation (members B–D) is unknown but constrained by the Eifelian age of the underlying shallow-marine fossiliferous deposits and by the Late Devonian to Early Mississippian age of the disconformably overlying Mangaqtaaq Formation and Kekiktuk Conglomerate. Based on these relationships, the age of the upper part of the Ulungarat Formation may range into the Late Devonian.

Mangaqtaaq Formation

The Mangaqtaaq Formation is a 135-m-thick, cyclic succession of black algal limestone, sandstone, and interbedded mudstone, that can be traced along strike for only 10 km (Figs. 3 and 7; Anderson and Watts, 1992). It is a distinctive, mappable unit within the Devonian to Mississippian succession. The lower half of the formation consists of 5–10-m-thick, cyclic repetitions of (1) algal limestone and interbedded sandstone with (2) thin intervals of recessive-weathering black mudstone. The upper half of the formation is dominated by black mudstone (Fig. 7). The succession is here formally named; the type section (Fig. 3, loc. D; Fig. 7) is in a small west-flowing tributary of the upper Kongakut River on the north side of Mangaqtaaq ridge (informal name) in the west half of Section 9, T5S, R38E, Demarcation Point A-4 quadrangle (69°1.4′N, 143°6.4′W). Mangaqtaaq is an Inupiat Eskimo word meaning “black color” (J. Nageak, University of Alaska Fairbanks, 1989, personal commun.). A detailed meter-by-meter description is in Anderson and Watts (1992).

Most of the formation is best exposed at the type section (Fig. 3, loc. D; Fig. 7), but the basal contact is covered by vegetation and not exposed. A good exposure of the basal contact can be seen 1.5 km west of the type section where the Mangaqtaaq Formation overlies the type section of the Ulungarat Formation (Fig. 3, loc. B) along a sharp contact. The contact is marked by a change from rose-red and green-gray mottled mudstone of the upper Ulungarat Formation to calcareous black mudstone of the Mangaqtaaq Formation. Beds above and below the contact are discordant with the contact. The contact is interpreted to be a low-angle disconformity, and the Mangaqtaaq Formation appears to onlap the disconformity surface. Two high-angle faults that locally offset the contact appear to be syn-Mangaqtaaq faults, indicating active faulting during Mangaqtaaq Formation deposition. The Mangaqtaaq Formation overlies the Ulungarat Formation where the older unit is at its maximum preserved thickness within the study area indicating a long-lived center of deposition (Fig. 3, loc. D). There is limited exposure of the upper contact of the Mangaqtaaq Formation beneath the Kekiktuk Conglomerate. The upper contact is interpreted as a very low-angle unconformity marking a zone of sediment bypass. See Supplemental Material (footnote 1) for a detailed description of the lithology and organization of the Mangaqtaaq Formation.

Age

The Mangaqtaaq Formation is of Late Devonian and/or Early Mississippian age based on stratigraphic position and interbedded plant fossils recovered from mudstone overlying sandstone channels in the lower 20 m of the formation. These include Sporangia, reminiscent of Tetrasylopteris (S. Mamay, U.S. Geological Survey, 1989, written commun.), indicating a broad Late Devonian to Early Mississippian age range. Based on regional relationships, we favor an early Late Devonian (Frasnian) age for the Mangaqtaaq Formation (see Discussion).

Kekiktuk Conglomerate

The Kekiktuk Conglomerate is an upward-fining and -thinning succession of chert and quartz-pebble to -cobble breccia and conglomerate, fine- to coarse-grained sandstone, and interbedded black shale. In the study area (Fig. 3, locs. E–G; Fig. 8), the formation is 0–70 m thick and contains coal, petrified wood, and plant fossils. In the footwall of the Aichilik Pass thrust exposed in the Little river valley and headwaters of Aichilik River (West Fork Valley succession), the Kekiktuk Conglomerate overlies the Romanzof formation with high-angle discordance (Fig. 3, loc. G, north of loc. B). In the Aichilik Pass and Kongakut River thrust sheets to the south (Continental Divide succession), the Kekiktuk Conglomerate unconformably overlies the Ulungarat Formation, or locally the Mangaqtaaq Formation (Fig. 3, locs. E and F, south of loc. D). The top of the formation is gradational with black mudstone and siltstone of the Kayak Shale. Correlation of the northern and southern successions and assignment to the Kekiktuk Conglomerate is based on lithologic similarity, the presence of distinctive purple-raspberry-colored chert pebbles in both areas, and stratigraphic position below the Kayak Shale. The purple-raspberry color is a discolored rim on light-gray chert pebbles.

In the Continental Divide succession, the Kekiktuk Conglomerate is 40–70 m thick. The basal contact is well exposed in the southwestern study area where it overlies the Ulungarat Formation (Fig. 3, loc. E). At this locality, a 2-km-long north-south exposure shows the contact is an erosional unconformity between the Kekiktuk Conglomerate and the Ulungarat Formation (Fig. 8). Though very low-angle discordance is possible, there is no obvious angular discordance.

To the north in the West Fork Valley succession, where the Kekiktuk Conglomerate directly overlies Romanzof formation (Fig. 3, loc. G; Figs. 5B and 5C), outcrops of Kekiktuk Conglomerate are discontinuous and generally less than 15 m thick. Locally, steep zones containing north-dipping fractures mark changes in relief on the unconformity surface. Beds of Kekiktuk Conglomerate, which fill lows on the unconformity surface, are not offset above the fractures. These relationships indicate that the fractures pre-date deposition of the Kekiktuk Conglomerate. See Supplemental Material (footnote 1) for a detailed description of the lithology and organization of the Kekiktuk Conglomerate.

Age

Kekiktuk Conglomerate of the Continental Divide succession in the study area is of pre-middle Tournaisian (Early Mississippian) age, based on the middle Tournaisian age of conodonts in the lowest interval of overlying Kayak Shale (A. Harris, 1991, written commun.). Plant fossils recovered from fine-grained intervals of Kekiktuk Conglomerate in the Continental Divide succession are Early Mississippian (Robert Spicer, 1991, personal commun.). Plant fossils are common on all surfaces where mudstone overlies clastic deposits.

Kayak Shale

The Kayak Shale is a micaceous, dark-gray to black, organic-rich, very finely fissile, locally silty mudstone that conformably overlies the Kekiktuk Conglomerate (Fig. 3). Beds of sandstone and argillaceous limestone are locally enclosed in the black shale. Coal is locally present in the lower Kayak Shale. Abundant organic matter consisting of woody and coaly material occurs throughout the succession (Utting, 1991).

Original depositional thicknesses of Kayak Shale are uncertain because it is structurally thickened or thinned by Brooks Range shortening. In the West Fork Valley succession, the formation is less than 100 m thick and composed of black shale with interbedded argillaceous limestone in the upper part. In the Continental Divide succession, the formation may be up to 200 m thick and includes intervals of sandstone and argillaceous limestone within the black shale. The top of the Kayak Shale is gradational with overlying platform carbonate of the Lisburne Group.

In the study area, the lowermost deposits of the Kayak Shale are, characteristically, organic-rich mudstone with abundant plant fossils and coal. These basal deposits are the same whether overlying the relatively planar upper surface of the Kekiktuk fluvial systems in the Continental Divide succession (Fig. 3, loc. E), or onlapping the West Fork Valley succession to the north (Fig. 3, loc. G).

The stratigraphic position of the Kayak Shale below limestone containing a marine fauna indicates a coastal setting. These relationships suggest that initial Kayak Shale deposition commenced in a low, swampy coastal-plain setting. Interbedding of Kekiktuk Conglomerate fluvial channel-fill deposits with black mudstone similar to Kayak Shale suggests the majority of coarse-grained clastic sediments were tied up in the fluvial system that crossed this coastal area. The overall upward-thinning and -fining of the Kekiktuk Conglomerate intervals suggest that non-marine terrigenous clastic dispersal systems retrograded in response to decreased amounts of sediment supplied to the system and/or relative sea-level rise.

Age

Based on trilobites, conodonts, and plant spores, the age of the Kayak Shale in the Continental Divide succession is of middle Tournaisian to Visean age (Early to Middle Mississippian). The lower Kayak Shale contains late Tournaisian trilobites (Linguaphillipsia) (Hahn and Hahn, 1993; Blodgett, 2008). The shales yield plant spores dated as Tournaisian to Visean (Utting, 1991). Conodonts in limestones in the lower Kayak Shale indicate an age of middle Tournaisian (probably late Kinderhookian) and in the upper Kayak Shale an age of Visean (early Meramecian). Conodonts from the base of the overlying Lisburne Limestone in the study area indicate a Visean age (early Late Meramecian; A. Harris, 1991, written commun.).

Provenance of the Ulungarat Basin Succession

The Middle Devonian to Mississippian terrigenous clastic Ulungarat Basin succession is compositionally similar throughout the study area. Based on field observations and petrographic study, the clastic rocks are composed of chert and quartz pebble breccia and conglomerate, lithic arenite, and siliceous mudstone. To characterize the fine-grained components of the Ulungarat Formation, 16 samples were selected for thin sections and point counting (Fig. 6 and Fig. S1 [footnote 1]). The selected samples were from the type section at location B (Figs. 3 and 6; ten samples, measured section 90A-31), location C (Fig. 3; five samples, measured section 90A-112, Fig. S1), and location A (Fig. 3; one sample, measured section 88A-1, Fig. S1). These data are reported in Table 1. Calculated modal percentages are reported in Table 2.

Study of thin sections shows that framework grains include chert, argillaceous chert, cherty argillite, and vein quartz. Abundant radiolarian ghosts are present in chert and argillaceous-chert grains. Results (Table 2) are plotted on QFL and QmFLt ternary diagrams, (Figs. 9A and 9B), where calculated detrital modes for Ulungarat Formation sandstones (N = 16) plot in a distinct group (Fig. 9C), indicating the provenance was a recycled orogenic belt with a major component of marine chert. Composition is not significantly different within the overlying Mangaqtaaq Formation and Kekiktuk Conglomerate, suggesting a common provenance.

The probable source terrane for the clastic rocks in the study area is the Romanzof formation, which was exposed to erosion to the north from Middle Devonian to Early Mississippian time. The Romanzof formation contains bedded to massive, gray, black, and white radiolarian chert with intercalated argillite that is lithologically identical to clasts in the Devonian to Mississippian succession.

Paleogeographic Evolution of the Ulungarat Basin

The stratigraphic record in the study area is interpreted to represent syn-rift and post-rift deposition across the Middle Devonian to Early Mississippian Ulungarat Basin rift-margin fault zone (Fig. 10). In the Continental Divide succession, deposition above the mid-Paleozoic regional unconformity began during Eifelian time (early Middle Devonian; Fig. 10A). The Ulungarat Formation (Du) was sourced from erosion of the Romanzof formation in the Romanzof orogenic highlands and deposited within active rift-margin, normal-fault–bounded basins. Shallow-marine, prodelta-front mudstones and siltstones of member A are overlain by non-marine flood-plain deposits of member B with red mottled paleosols. Sandstones of member B deposited by meandering streams coarsen and thicken upward documenting progradation. Sandstones and conglomerates of member C were deposited in entrenched channels within large erosional scours characteristic of the upper alluvial fan environment. Progradational stacking of increasingly coarse-grained deposits and low-angle unconformities suggest an active tectonic regime. The Ulungarat Formation is separated from the overlying lacustrine deposits of the Mangaqtaaq Formation (Dm) by a very low-angle disconformity. Taken together, the Ulungarat and Mangaqtaaq formations record the marine to non-marine progradation of a terrigenous clastic system into the Ulungarat Basin, followed by local lacustrine deposition in a hydrologically closed, fault-controlled topographic low. Deposition of the Mangaqtaaq Formation directly above the maximum preserved thickness of the Ulungarat Formation, while elsewhere Ulungarat Formation rocks remained exposed, indicates continued local structurally controlled subsidence of the Ulungarat Basin depocenter. Together, these relationships suggest a restricted basin created by faulting and tilting of the Ulungarat Formation.

The earliest deposition of sandstones and conglomerates of the Kekiktuk Conglomerate (Mkt-2) began in the Continental Divide succession in pre-middle Tournaisian time as a coarse-grained, retrogradational braided fluvial system (Fig. 10B). This system marked renewed deposition over a low-angle bypass unconformity, where active channels eroded into the underlying Ulungarat (Du) and Mangaqtaaq (Dm) formations. Composition and transport direction indicate that the probable source of these clastic rocks continued to be erosion of the Romanzof formation along the basin margin to the north, but motion on normal faults had ceased by this time. Coarse-grained clastic detritus and the black shales of the Kekiktuk Conglomerate are interpreted to have been deposited within the coastal-plain fluvial system, representing channel and interchannel deposits, respectively. Deposits of the retrograding Kekiktuk Conglomerate fluvial system thin and fine upward and to the north.

By middle Tournaisian time, coastal plain and marine shales of the Kayak Shale (Mk) had transgressed the Kekiktuk Conglomerate (Mkt-2) in the Continental Divide succession, recording coastal retreat and drowning of a low-energy paleo-shoreline in response to passive-margin subsidence (Fig. 10C). North of the rift basin margin, the Kekiktuk Conglomerate (Mkt-1) of the West Fork Valley succession, consisting of thin, laterally discontinuous, locally derived debris flow, colluvial, and fluvial deposits, was strongly influenced by erosional relief on the underlying unconformity surface. Normal faults were no longer active at this time, and the entire rift margin was subsiding. The conformable contact between the Kekiktuk Conglomerate (Mkt-2) and black coastal-plain and marine muds of the Kayak Shale (Mk) of the Continental Divide succession is an undulating planar surface along which abundant plant fossils, coal, and black mudstone are present, suggesting a low-energy, wet, low-relief coastal plain. The West Fork Valley succession onlaps the Romanzof formation on the mid-Paleozoic regional unconformity surface.

Brooks Range Deformation in the Study Area

The study area is within the parautochthon of the northeastern Brooks Range (Fig. 1), a thick-skinned fold-thrust belt characterized by large regional anticlinoria cored by pre-Middle Devonian “basement” (Wallace and Hanks, 1990). The parautochthon developed in the Cenozoic as a northward-propagating duplex with a floor thrust at mid-crustal depths and a roof-thrust in Kayak Shale (Wallace and Hanks, 1990; Hanks, 1993; Hanks et al., 1994). The Aichilik Pass and Kongakut River thrust sheets (Fig. 3) form a smaller-scale, north-vergent early Cenozoic duplex on the flank of the southernmost anticlinorium in the Romanzof Mountains, with a basal detachment within lower Ulungarat Formation (Du) marine shale (member A) and a roof thrust in the Kayak Shale (Mk). Two schematic cross sections illustrate stratigraphic and structural relationships in the study area (Fig. 11, X–X′ and Y–Y′).

Cross section X–X′ (Fig. 11) shows relationships in the southwestern study area, where the Ulungarat Formation (Du) is in direct depositional contact with Romanzof formation (Or). Cenozoic deformation above the deep detachment within the Romanzof formation “basement” tilts and folds the Middle Devonian to Mississippian cover section on the south flank of the regional anticlinorium.

Cross section Y–Y′ (Fig. 11) shows stratigraphic and structural relationships across the central study area where the Middle Devonian to Mississippian clastic succession is thickest. Here, the early Cenozoic duplex is well developed, consisting of two thrusts, the Aichilik Pass thrust and the Kongakut River thrust, that define two south-dipping, thin-skin thrust sheets of the Continental Divide succession. The Aichilik Pass thrust places Middle Devonian Ulungarat Formation (Du) over thin Mississippian Kekiktuk Conglomerate of the West Fork Valley succession (Mkt-1) in surface exposures (Figs. 3 and 11). Ulungarat Formation (Du) and Mangaqtaaq Formation (Dm) rocks are not found north of the Aichilik Pass thrust. A concealed basin-bounding normal fault is interpreted to lie beneath the ramp of the Aichilik Pass thrust. Based on map relationships to the east, the hanging wall of the Aichilik Pass thrust translated northward ∼3 km up a 15° ramp, localized by the buttress of the basin-bounding normal fault zone.

Mechanical stratigraphy of the Middle Devonian to Lower Mississippian succession changes across the Aichilik Pass thrust. The Continental Divide succession is mechanically decoupled from the basement by a detachment in the lower Ulungarat Formation (Du) and deforms as a thin-skin duplex. North of the Aichilik Pass thrust, thin Kekiktuk Conglomerate of the West Fork Valley succession (Mkt-1) is deposited directly on Romanzof formation (Or), deforming with basement as a single mechanical unit. The change in mechanical stratigraphy is interpreted to coincide with termination of Ulungarat marine shale at the basin-margin normal fault, forcing the detachment in the basal Ulungarat Formation to ramp up into the Kayak Shale (Fig. 11). Slip on the younger mid-crustal detachment that formed the large anticlinorium in the Romanzof formation is interpreted to have folded the older, shallower duplex.

Apatite fission-track analysis on samples collected in the study area indicates a cooling age of ca. 59 Ma (late Paleocene; P. O'Sullivan, 1991, written commun.). Long track lengths indicate rapid cooling, probably due to uplift and erosional unroofing. Similarity in ages on samples from the Kongakut River thrust sheet, the Aichilik Pass thrust sheet and the underlying Romanzof formation suggest that cooling ages date formation of the deep-seated anticlinorium (Anderson, 1993). These data are in agreement with a similar age for rapid uplift and unroofing of parautochthonous rocks at nearby Bathtub Ridge (O'Sullivan et al., 1993).

The Aichilik Pass thrust and Kongakut River thrust are low-displacement faults (∼3 km). Major early Cenozoic Brooks Range shortening occurred on the deep faults resulting in the large regional anticlinoria that underlie the parautochthon. Cenozoic structures of the northeastern Brooks Range are consistent in character and orientation throughout the Ulungarat Basin succession. There is no evidence in the study area to support a possible interpretation of either Devonian–Mississippian or Cenozoic strike-slip faulting as suggested by Strauss et al. (2019).

The Ulungarat Basin Rift Margin

The stratigraphic record in the study area is interpreted to represent syn-rift and post-rift deposition of locally derived terrigenous clastic rocks across a Middle Devonian to Early Mississippian rift basin-margin fault zone. The West Fork Valley succession to the north was deposited on the uplifted basin margin, whereas the Continental Divide succession was deposited in the Ulungarat Basin, an extensional rift basin south of a basin-margin normal fault system. Both terrigenous clastic successions are separated from underlying deformed Romanzof formation by a profound angular unconformity, the mid-Paleozoic regional unconformity, and are conformably overlain by passive margin shelfal carbonates of the Lisburne Group. Together, the successions form a southward-thickening clastic wedge that increases in thickness from ∼100 m on the uplifted margin to ∼900 m in the Ulungarat Basin. This change in thickness is due to both the presence of the Ulungarat and Mangaqtaaq formations and an increase in thickness of the Kekiktuk Conglomerate and Kayak Shale. The depositional record indicates southward progradation of depositional systems into a marine to non-marine basin followed by Early to Middle Mississippian (early Tournaisian to Visean) transgression. The Ulungarat and Mangaqtaaq formations are interpreted to record early Middle to Late Devonian syn-rift deposition, whereas the overlying Kekiktuk Conglomerate and Kayak Shale are interpreted to record Mississippian post-rift passive-margin subsidence.

A mid-Paleozoic tectonostratigraphic chart (Fig. 12) summarizes the age relationships of the Middle Devonian to Lower Mississippian deposits of Arctic Alaska to document the evolution of the continental margin. The chart was constructed using published fossil and radiometric age constraints (see Table S1 [footnote 1]). Middle Devonian to Lower Mississippian chert- and quartz-pebble conglomerates found across Arctic Alaska were deposited in different tectonic settings. The Ulungarat Formation was deposited in a proximal syn-rift basin, while the slightly younger Kanayut delta was deposited in a more distal continental margin setting during rapid, early passive-margin subsidence. Kekiktuk Conglomerate deposition in fluvial-cut valleys of the northeastern Brooks Range and Kekiktuk Formation deposition in fault-bounded basins of the North Slope was a response to longer-term passive-margin subsidence and relative sea-level rise across the broad rift-margin platform.

Age of the Mid-Paleozoic Regional Unconformity

In the Ulungarat Basin, Middle Devonian clastic rocks unconformably overlie deformed Romanzof formation on the mid-Paleozoic regional unconformity, constraining age of the unconformity, and end of Romanzof deformation, to the Emsian/Eifelian (Early/Middle Devonian) boundary (Fig. 12). Middle Devonian post-orogenic rifting resulted in early transgression of the mid-Paleozoic regional unconformity in Ulungarat Basin outboard of the rift margin, where the surface is locally overlain by syn-rift strata of the Ulungarat Formation of Eifelian age. Inboard of the rift margin, erosion persisted until the unconformity was ultimately transgressed by post-rift passive-margin strata of the Kekiktuk Conglomerate of latest Tournaisian age. North of Ulungarat Basin, deformed rocks of the Romanzof orogeny were exposed to erosion from Eifelian to latest Tournaisian time, a period of ∼50 m.y., encompassing the combined effects of Romanzof orogenic uplift and subsequent rift-related footwall uplift of the rift-basin margin (Fig. 12). The age of the mid-Paleozoic regional unconformity is defined by the oldest rocks that lie above it; therefore, the age of the unconformity in the northeastern Brooks Range and North Slope is Eifelian, despite being onlapped by much younger Mississippian strata to the north.

In the northeastern Brooks Range, the mid-Paleozoic regional unconformity is described as a low-relief fluvial cut surface (LePain et al., 1994). During the Devonian, interfluvial areas lacked vegetation because land plants had only colonized lowland areas (Schumm, 1968; Davies and Gibling, 2010; Miall, 2010). Resulting rapid erosion of highlands in Middle to Late Devonian time supplied vast quantities of terrigenous clastic detritus to depositional systems to the south. The shallow-marine to non-marine Ulungarat Formation is the earliest depositional record of this erosion. By Early Mississippian time, erosion of highlands reduced the source area to a broad low-relief surface, and transgression resulted in unconformable infilling of low areas by thin fluvial strata of Kekiktuk Conglomerate followed by Kayak Shale. The Middle to Late Devonian Okpilak batholith (381 ± 10 Ma; Dillon et al., 1987b) was rapidly exhumed (Gottlieb et al., 2014), exposed to erosion, then buried during Visean time by deposition of Kekiktuk Conglomerate and Kayak Shale.

Middle to Late Devonian Syn-Rift Deposition

The Ulungarat Basin provides a critical tectonic and depositional link between the syn-rift and passive margin successions, as well as between the Middle to Upper Devonian autochthonous and allochthonous successions (Fig. 12). Ulungarat Basin lies along the northern margin of a system of marine extensional basins that underlay the complex southern rift margin of Arctic Alaska in Middle to Late Devonian time. Rifting began in Ulungarat Basin immediately after culmination of the Romanzof orogeny in early Middle Devonian (Eifelian) time, as evidenced by stratigraphic and structural relationships. Deposition of the Ulungarat and Mangaqtaaq formations overlaps with syn-rift deposition of the Beaucoup Formation, Ambler Group volcaniclastics, and Angayucham interpillow carbonates. Beaucoup Formation of Givetian to Frasnian (latest Middle to early Late Devonian) age is locally gradational with overlying Hunt Fork Shale of Frasnian-Famennian (Late Devonian) age at the base of the Ellesmerian megasequence. The base of the Beaucoup Formation is truncated by thrusting, and therefore its lower age limit is not known. Beaucoup strata are interpreted as distal marine equivalents of the Ulungarat Basin rift succession, with the late-Frasnian age of basal Hunt Fork Shale marking onset of post-rift passive margin deposition.

The age range of plant fossils in the Mangaqtaaq Formation is very broad and only constrains deposition to sometime in Late Devonian and/or Early Mississippian time. Structural and depositional affinities, along with coincidence of the thickest Mangaqtaaq and Ulungarat sections, argue that Mangaqtaaq deposition immediately followed Ulungarat deposition as part of the same syn-rift basin. Therefore, we have interpreted Mangaqtaaq deposition as Frasnian and end of Mangaqtaaq deposition as latest Frasnian to correspond to the Beaucoup/Hunt-Fork boundary (Fig. 12). In our age interpretation, the Mangaqtaaq Formation of Ulungarat Basin correlates with the Beaucoup Formation of the Endicott Mountains allochthon, not the Endicott Group. Deposition of the post-rift allochthonous Kanayut delta is interpreted to correspond to a bypass unconformity between Mangaqtaaq Formation and Kekiktuk Conglomerate in Ulungarat Basin.

In the Ikpikpuk-Umiat and Meade extensional basins of the western North Slope, the deep section is speculated to be Devonian in age, based on correlation of seismic-reflection lines and two well penetrations (Grantz and May, 1988; Dumoulin, 2001; Saltus et al., 2002). Seismic-reflection data image tilted deep basin strata truncated by a low-angle unconformity and overlain by basal strata of the Ellesmerian megasequence (Grantz and May, 1988; Mauch, 1989). The Topagoruk #1 well, located on the northern edge of Ikpikpuk-Umiat Basin, penetrated and cored 137 m (450 ft) of unmetamorphosed Middle Devonian clastic strata at its base with reported dips of 35°–60° (Collins, 1958). There is a lack of consensus whether reported dips are the result of (1) Late Devonian to Early Mississippian folding (Grantz et al., 1990; Moore et al., 1994; Dumoulin, 2001) or (2) Middle to Late Devonian rotation on listric normal faults (Grantz and May, 1988; Saltus et al., 2002). There is also disagreement whether these strata should be assigned to (1) Romanzof basement (Franklinian sequence of Kirschner and Rycerski, 1988), (2) an early deformed pre-Ellesmerian extensional sequence (Grantz et al., 1990; Dumoulin, 2001), or (3) an early extensional sequence either below or within the basal Ellesmerian sequence (Eo-Ellesmerian of Grantz and May, 1988; Saltus et al., 2002).

Because of its relevance to the age of mid-Paleozoic deformation, core from the Topagoruk #1 well was reexamined to clarify relationships described by Collins (1958). The core consists of chert conglomerate and sandstone interbedded with mudstone deposits containing leaf impressions, the character and organization of which indicate a succession of fluvial and flood-plain deposits. Plant fossils are similar to those observed in flood-plain deposits of member B of the Ulungarat Formation. In cross-stratified deposits, depositional horizontal is best approximated by the most gently dipping cross-laminae, rather than the steeply dipping, coarse-grained trough cross-stratified deposits. Applying this criterion to the cross-stratified sandstone beds in the Topagoruk #1 well, the dip of depositional horizontal is interpreted to be between 12° and 24°, indicating a low-angle discordance with overlying beds (Fig. 13). Furthermore, the Middle Devonian Topagoruk #1 succession lacks the penetrative structures and low-grade metamorphism that characterize the majority of lower Paleozoic basement rocks throughout the North Slope subsurface. Vitrinite reflectance values from the well are consistent with burial of the Devonian strata under the same thermal regime as overlying strata, with no thermal discontinuity (Magoon and Bird, 1988). The lack of significant angular discordance, penetrative structures, and thermal discontinuity suggests a history for the Middle Devonian strata not significantly different from overlying strata. On this basis, we conclude that this succession overlies and postdates the deformation and metamorphism of the basement complex. We therefore concur with Saltus et al. (2002) that these Middle Devonian strata are syn-rift basin deposits that date the Middle Devonian onset of rifting in the western North Slope.

Late Devonian to Mississippian Post-Rift Passive Margin Deposition

Passive-margin accommodation forms as a result of thermal and/or flexural subsidence following regional extension, rifting, and breakup of a continental margin (Steckler et al., 1988). Regional crustal extension associated with rifting ends with continental breakup, onset of spreading, and emplacement of oceanic crust (Bond and Kominz, 1988). In Arctic Alaska, initiation of passive-margin subsidence is associated with deposition of the Ellesmerian megasequence interpreted to coincide with onset of spreading in the Angayucham oceanic basin (Moore et al., 1994).

Both allochthonous and autochthonous Endicott Group deposits are part of the passive margin succession. Although clastic deposits of the Kanayut delta are generally dated as Famennian to early Tournaisian, the basal Hunt Fork Shale contains Frasnian fossils. Hunt Fork Shale conformably overlies Beaucoup Formation of Givetian to Frasnian age. The Beaucoup Formation displays characteristics consistent with a syn-rift origin, suggesting that the rift-drift transition is latest Frasnian (Fig. 12). By earliest Mississippian time, a major transgression of the Arctic Alaska margin was underway. The base of the Kayak Shale marks the non-marine to marine transition, which is time-transgressive and becomes younger to the north, a pattern also evident in the age of the base of the Lisburne Group carbonates (Fig. 12).

The Ulungarat Basin and allochthonous Endicott Group deposits are genetically related as evidenced by close similarities in lithology, provenance, depositional organization, sediment transport direction, and stratigraphic position. Both record south-to-southwest prograding, coarse-grained fluvial depositional systems that were succeeded by the transgressive Kayak Shale. The allochthonous Endicott Group succession and Ulungarat Basin share a chert-rich provenance, with clast composition and sediment transport direction suggesting a common source in strata uplifted by the Romanzof orogeny (Nilsen and Moore, 1984; Anderson, 1987, 1993). The Ulungarat and Mangaqtaaq formations are older, however, and interpreted as the earlier up-dip syn-rift record of the depositional system that ultimately evolved into the massive Kanayut delta. Deposition of the Kanayut delta is interpreted to correspond to a bypass unconformity above the Mangaqtaaq Formation in the Ulungarat Basin (Fig. 12).

The Kekiktuk Conglomerate and Kayak Shale of the West Fork Valley succession are typical of parautochthonous Endicott Group exposed throughout most of the northeastern Brooks Range. Although thinner and younger to the north, Kekiktuk Conglomerate and Kayak Shale of Ulungarat Basin (Continental Divide succession) and West Fork Valley succession are clearly related and interpreted to represent originally laterally continuous deposition across a basin margin, now foreshortened by north-vergent Cenozoic thrusting.

The Endicott Group autochthonous succession is also exposed in Doonerak window of the central Brooks Range (Fig. 1), where Kekiktuk Conglomerate is unconformably deposited on deformed pre-Middle Devonian basement and overlain by basal Kayak Shale of middle Tournaisian age. The Doonerak succession was deposited on a basement high contemporaneous with deposition of Kekiktuk Conglomerate in Ulungarat Basin. A balanced north-south regional cross section across the northeastern Brooks Range parautochthon shows that when Cenozoic shortening on large regional anticlinoria is removed, the Ulungarat Basin restores ∼115 km south of its present position (Hanks et al., 1994). This places the basin on trend with the footwall of the Endicott Mountains allochthon in the central Brooks Range to the west, where equivalent rift basins may be concealed.

Throughout the northeastern Brooks Range north of the Ulungarat Basin, Kekiktuk Conglomerate of late Tournaisian age is unconformably deposited on deformed pre-Middle Devonian basement and overlain by Kayak Shale of Visean age (Fig. 12). Within the Ulungarat Basin, Kekiktuk Conglomerate of early Tournaisian age is unconformably deposited on undeformed Ulungarat or Mangaqtaaq formations and overlain by Kayak Shale of middle Tournaisian age. Kekiktuk Conglomerate in Ulungarat Basin is not constrained by fossils but must be pre-middle Tournaisian in age based on the overlying Kayak Shale. The boundaries between these units are younger than facies equivalents in the Endicott Mountains allochthon and older than facies equivalents in the northeastern Brooks Range and North Slope (Fig. 12). This northward younging of units is interpreted to reflect progressive onlap of the margin during passive-margin subsidence.

The western North Slope extensional basins were reactivated in Early Mississippian time, resulting in local inversion of basin-fill and strike-slip faulting extending into mid-Mississippian time. The eastern North Slope lacks older extensional basins and was experiencing erosion prior to earliest Visean time when faults at the Endicott field become active and Kekiktuk Formation was deposited (Melvin, 1993). This later episode of reactivation and extensional faulting is not part of the normal evolution of a passive margin and likely reflects intra-plate deformation associated with a younger tectonic event. While the Ulungarat Basin shares the history of Middle to Late Devonian regional extension common throughout Arctic Alaska, the basin shows no evidence of Mississippian compression or transpression.

Widespread subsidence and passive margin deposition across the Arctic Alaska continental margin in Late Devonian to Middle Mississippian time was the direct result of rifting in Middle to Late Devonian time. Although the stratigraphic record of this rifting event is obscured by Brooks Range deformation and foreland burial, the syn-rift to post-rift tectonic transition is preserved and exposed in the Ulungarat Basin succession. This important succession constrains the timing of key events in the tectonic transition from the Romanzof contractional orogen, to the Arctic Alaska rift, and, ultimately, the Ellesmerian passive margin.

  • Ulungarat Basin is composed of both syn-rift and post-rift deposits.

    • Syn-rift Ulungarat Formation overlies deformed Romanzof formation at the mid-Paleozoic regional unconformity, limiting the Romanzof orogeny to pre-Eifelian time.

    • Initial Ulungarat Formation deposits are shallow marine rocks of Eifelian age, suggesting early connection with marine rift basins of the south Arctic Alaska margin.

    • A disconformity with very low-angle discordance separates the Ulungarat and Mangaqtaaq formations.

    • Syn-rift Mangaqtaaq Formation lacustrine deposits, interpreted as Late Devonian in age, suggest deposition in a restricted structural low.

    • Post-rift transgressive deposition of the Kekiktuk Conglomerate of early Tournaisian age and Kayak Shale of middle Tournaisian age is older and thicker in the Ulungarat Basin than on the rift margin to the north.

  • Ulungarat Formation and Mangaqtaaq Formation are elevated to formal formation status.

  • Similarities of composition, stratigraphy, and plant fossils between Ulungarat Formation and Middle Devonian clastic strata penetrated in the Topagoruk #1 well suggest regional extension in Arctic Alaska was broadly coeval and began in Middle Devonian time.

  • Ulungarat Basin and its northern rift-basin margin were transported ∼115 km north by thick-skinned Cenozoic Brooks Range thrusting.

    • Compressional structures in Ulungarat Basin strata are consistent in character and orientation throughout, indicating only Cenozoic Brooks Range deformation.

    • Ulungarat Basin does not show any evidence of Late Devonian to Early Mississippian compressional or transpressional deformation.

  • Middle Devonian to Lower Mississippian conglomerates and sandstones of Arctic Alaska were deposited in different tectonic settings during a period of rapid tectonic transition.

    • Syn-rift deposition of Ulungarat Basin occurred in Middle to Late Devonian time just seaward of the Arctic Alaska rift margin.

    • Post-rift deposition of allochthonous Endicott Group occurred in Late Devonian to Early Mississippian time outboard of the Arctic Alaska rift margin.

    • Passive margin transgressive deposition of parautochthonous Endicott Group of the northeastern Brooks Range occurred in Early to Middle Mississippian time on the proximal rift shoulder inboard of the rift basins.

    • Syn- and post-rift deposition of autochthonous Endicott Group in faulted North Slope grabens occurred in Middle Devonian to Middle Mississippian time.

  • The long-lived mid-Paleozoic regional unconformity is early Middle Devonian in age (Emsian/Eifelian boundary), underlain by deformed rocks of the Romanzof orogeny and overlain by both syn-rift and post-rift successions.

    • In the North Slope and northeastern Brooks Range, this unconformity surface represents up to ∼50 m.y. of erosion and/or non-deposition.

    • Devonian to Lower Mississippian chert- and quartz-pebble conglomerates and sandstones found across Arctic Alaska are similar in composition and were likely sourced from erosion of both the Romanzof orogenic highlands and the Middle Devonian to Early Mississippian rift shoulder.

This study is part of a Ph.D. dissertation by Anderson on stratigraphic variation across a rift-basin margin and implications for fold and thrust geometry. Anderson appreciates the inspiration and guidance of Dr. Peter J. Coney (University of Arizona), who introduced her to the complex questions concerning the Devonian evolution of Arctic Alaska, and Dr. Wesley K. Wallace (University of Alaska, Fairbanks), who introduced her to the structural complexity of the northeastern Brooks Range, Alaska. Gil Mull is thanked for many discussions in the field and his encouragement to write this paper. Meisling would also like to acknowledge Gil Mull for sharing his knowledge of Brooks Range geology and Elizabeth Miller for her mentorship and for stoking his interest in the Devonian tectonics of the Arctic. We thank Tom Moore for reviewing an early version of this paper. His constructive review and thought-provoking feedback greatly improved the manuscript. This study benefited from discussions with Ken Bird, Keith Crowder, Julie Dumoulin, Larry Lane, David LePain, and Keith Watts. We thank the following experts for their identifications: plant fossils—S. Mamay (USGS) and Robert Spicer (University of Oxford [now Open University UK]); algae—M. Mickey (Micropaleo Consultants) and W. Nassichuk (Geological Survey of Canada); plant spores—J. Utting (Geological Survey of Canada); and conodonts—A. Harris (USGS). Robert Blodgett and his global paleontological colleagues provided important data in the identification of rare early Paleozoic megafaunal remains. We thank Science Editor David Fastovsky and Associate Editor Francesco Mazzarini (Geosphere), as well as Jaime Toro and three anonymous peer reviewers for their thoughtful reviews and numerous suggestions for improvement of the manuscript.

Anderson gratefully acknowledges financial support from the Tectonics and Sedimentation Research Group, Department of Geology and Geophysics, University of Alaska, Fairbanks (grants from ARCO Alaska, ARCO Research, BP Exploration [Alaska], Chevron, Conoco and Phillips Petroleum [now both part of ConocoPhillips], Elf Aquitaine, Exxon and Mobil Exploration and Producing [now both part of ExxonMobil], Japan National Oil Corporation, Marathon Oil, Murphy Oil, Shell Western Exploration and Producing, Texaco, and Unocal). Additional financial support to Anderson from the American Association of Petroleum Geologists, Alaska Geological Society, Amoco, Geological Society of America, Geist Fund of the University of Alaska Fairbanks Museum, and Sigma Xi is gratefully acknowledged. Special thanks to the U.S. Fish and Wildlife Service for allowing the project to purchase helicopter time.

1Supplemental Material. Describes the organization, sedimentology, and depositional environments of the Ulungarat Basin succession, including description of type sections of the Ulungarat and Mangaqtaaq formations. Table S1 documents published fossil and radiometric age constraints used to construct the mid-Paleozoic tectonostratigraphic chart (Fig. 12), including basis for age assignment and list of source references. A reference list of all sources cited in Table S1 is included. Please visit https://doi.org/10.1130/GEOS.S.14781495 to access the supplemental material, and contact editing@geosociety.org with any questions.
Science Editor: David E. Fastovsky
Associate Editor: Francesco Mazzarini
Gold Open Access: This paper is published under the terms of the CC-BY-NC license.