Tectonic and Paleogeographic Overview of the Basin

Subsidence of the Western Canada Foreland Basin was initiated in the Jurassic as the result of isostatic flexure of underlying cratonic lithosphere under the load of the tectonically thickened Cordilleran fold-and-thrust belt to the west (Price, 1994; Evenchick et al., 2007). The earliest preserved foreland strata were deposited from the Late Jurassic (Kimmeridgian) to Early Cretaceous (Valanginian), when sediment was supplied to the Canadian portion of the basin by both local rivers and a continent-scale, north-flowing drainage system with headwaters in the southwestern and possibly also the southeastern United States (Williams and Stelck, 1975; Wright et al., 1994; Raines et al., 2013). In the Berriasian to Barremian, diminished tectonic activity in the Rocky Mountain Cordillera, resulted in widespread erosion and isostatic uplift of Upper Jurassic to Lower Cretaceous sediments in the foredeep (Leckie and Cheel, 1997; Leier and Gehrels, 2011). Subsidence of the Western Canada Foreland Basin resumed in the Aptian, accompanied by gradual marine flooding by a southward-advancing arm of the Polar Ocean. One or more continent-scale, north- and westflowing river systems delivered sediment to the basin from headwaters in the Canadian Shield, the Appalachians, and the Cordillera (Benyon et al., 2014; Blum and Pecha, 2014).

At about the Albian-Cenomanian boundary (about 100.5 Ma), a southward-encroaching embayment of the Polar Ocean (the Mowry Sea) merged with a northward-encroaching arm of the Gulf of Mexico to form the early Greenhorn Sea (Williams and Stelck, 1975). This seaway continued to widen through the late Cenomanian to a maximum extent in the early Turonian, before progressive eustatic sea-level fall caused the seaway to narrow to a minimum in the late Turonian and earliest Coniacian, as recorded by rocks of the Cardium Formation (e.g., Nielsen et al., 2008; Shank and Plint, 2013; Walaszczyk et al., 2014). Earliest Coniacian regressive sandstones in the upper part of the Cardium Formation were buried by marine mudstone of the early to middle Coniacian Muskiki Member of the Wapiabi Formation. Muskiki strata record the onset of the Niobrara Cycle (Kauffman, 1977), which is expressed as a major transgression that drove the shoreline so far to the west that no near-shore deposits are preserved in outcrop sections exposed in the Alberta or British Columbia (BC) foothills (Stott, 1963, 1967; fig. 1). In late Coniacian time, the broadly regressive, siltstone- and sandstone-dominated Marshybank Member was deposited along the western margin of the basin. The slightly younger Bad Heart Formation was deposited in the northeastern part of the basin, and is confined to the Peace River Plains (Stott, 1963, 1967; Plint et al., 1990; Donaldson et al, 1998). Late Coniacian regression was followed by major regional transgression in latest Coniacian and early Santonian time, leading to widespread deposition of marine mudstone across Alberta and BC; this transgression coincided with a period of renewed flexural subsidence of the foredeep of the foreland basin (Nielsen et al., 2008; Hu and Plint, 2009; Plint et al., 2012a).

Lithostratigraphy

The early history of stratigraphic investigation of Upper Cretaceous rocks in Alberta was thoroughly reviewed by Stott (1963, 1967). The present study is focused on Coniacian rocks that form part of the Wapiabi Formation, a unit that was originally recognized and named by Malloch (1911). The Wapiabi Formation comprises up to about 650 m of marine mudstone with minor intercalations of sandstone and siltstone. Stott (1956, 1963, 1967) showed that the Wapiabi Formation could consistently be divided into seven members, each characterized by a distinct suite of lithologies that could be traced along most of the Rocky Mountain Foot-hills from NE BC to the Montana border (figs. 2, 3; fig. 3 inserted after p. 52). The lowest unit, the Muskiki Member, rests disconformably on the underlying Cardium Formation and comprises primarily offshore marine mudstone.

The boundary between the Muskiki and overlying Marshybank Member is typically a rapidly gradational to sharp contact between mudstone and siltstone and overlying, intensely bioturbated siltstone and silty sandstone. Toward the north, particularly north of the Athabasca River, the siltstone-dominated Marshybank Member undergoes a lateral facies change to include clean, well-sorted sandstone as well as siltstone and minor mudstone. Even further to the NW, in BC, the Marshybank includes not only near-shore sandstone but also a unit of terrestrial deposits (Plint and Norris, 1991). Stott (1967) did not extend the term “Marshybank” to describe these northern, sandstone-rich rocks, but instead chose to assign them to the “Bad Heart” Formation, inferring that sandy rocks in the foothills correlated with the type Bad Heart Formation that is exposed about 150 km to the NE on the Smoky River in the Peace River Plains. In the latter area, the Bad Heart Formation comprises fine-grained sandstone and ooidal ironstone with minor dark silty claystone.

A change of lithostratigraphic terminology occurs at the Canada—U.S. border: rocks of the Cardium and Wapiabi formations in Alberta being broadly correlative with the Ferdig and Kevin members of the Marias River Formation in Montana (Cobban et al., 1976; Nielsen et al., 2003; figs. 2, 3; figs. 3–13 inserted after p. 52).

Biostratigraphic Studies

Stott (1963, 1967) summarized existing biostratigraphic information regarding the age of the Wapiabi Formation. Stott reported that the lowest part of the Muskiki Member contained Scaphites (S.) preventricosus Cobban and Inoceramus deformis Meek. Stott used the stratigraphical interpretation of Jeletzky (who followed the traditional German subdivision) and dated these two taxa to the late Turonian, although he (Stott, 1963) did note that Jeletzky (personal commun.) considered that they might be of earliest Coniacian age (as treated at that time in the U.S.; see e.g., Seitz, 1959). The higher part of the Muskiki contained Scaphites (S.) ventricosus Meek and Hayden and Inoceramus involutus Sowerby, indicative of a Coniacian age. Neither I. involutus nor S. (S.) ventricosus were found to range up into the Marshybank Member (Stott, 1963). The Marshybank Member contained Scaphites (S.) depressus Reeside, which Stott did not find below the base of the member. At the time of Stott's studies, S. (S.) depressus was considered by Jeletzky (see also discussion in Obradovich and Cobban, 1975) to indicate an early Santonian age, but that zone was later reassigned to the upper Coniacian (Kennedy and Cobban, 1991). Collom (2001) undertook a litho- and biostratigraphic study of the Wapiabi Formation and equivalent strata between Highwood River in the south and the lower Smoky River in the north. Collom's correlation of depositional cycles along the Foothills was, however, made without reference to subsurface data, and his stratigraphic sections contain insufficient detail to be related to our own observations. Moreover, Collom's inoceramid taxonomy, and consequently his stratigraphic interpretation, cannot be confirmed in most of the cases (see companion paper by Walaszczyk et al., this issue).

FIG 1.

Paleogeographic reconstruction of North America for the middle Coniacian (Scaphites (S.) ventricosus time). This map is representative of the paleogeography during deposition of the Muskiki Member of the Wapiabi Formation when the Coniacian transgression was at approximately its greatest extent. The study area in Alberta and Montana is indicated. (Map adapted from R. Blakey,  http://cpgeosystems.com).

In Montana, Cobban et al. (1976) recognized ammonite and bivalve zones in the Ferdig and Kevin members of the Marias River Formation, allowing them to establish a broad correlation with the Cardium Formation and the Muskiki and Marshybank members of the Wapiabi Formation in Alberta. Cobban et al. (2005) made a detailed study of a thin (typically 10–20 cm) carbonate-cemented conglomerate bed (Bed 100 of the Kevin Member; Cobban et al., 1976) that, because it was so distinctive and widespread, was named by Cobban et al. (1959) the MacGowan Concretionary Bed. The bed contains small, wellrounded chert pebbles as well as reworked phosphate nodules and molluscan fossils. This bed lies at, or just above, the boundary between middle and upper Coniacian strata. A second bed of green-stained phosphate nodules, some of which are bivalve steinkerns, is located 3.5 m above the MacGowan Bed at the Type Section of the Kevin Member (Bed 108 of Cobban et al., 1976). Cobban et al. (2005) speculated that the MacGowan Bed in Montana might be correlative with the erosion surface documented beneath the Bad Heart Formation in Alberta, 850 km to the NW (Plint et al., 1990; Donaldson et al., 1998, 1999).

FIG. 2.

Summary of stratigraphic schemes that have been applied to Upper Cretaceous strata in Alberta, British Columbia and northern Montana. The far left column shows the allostratigraphically defined limits of the Cardium alloformation, the base of which (surface E1) is placed below the lithostratigraphically defined position of Stott (1963, 1967).

Previous Work

Regional allostratigraphic correlation of Coniacian rocks was initially conducted in northwestern Alberta and adjacent BC, where it was shown that upward-coarsening successions, bounded by marine flooding surfaces, could be traced for several hundred km in the subsurface, and also correlated into outcrop (Norris, 1989; Plint, 1990; Plint et al., 1990; Plint and Norris, 1991). These studies showed that the erosion surface that defined the top of the “Bad Heart” Formation in the foothills could be correlated with an erosion surface that defined the base of the type Bad Heart in the Peace River Plains. Plint et al. (1990) therefore proposed that Stott's term “Marshybank” be applied to all siltstone and sandstone facies overlying the Muskiki mudstones, throughout the foothills (fig. 2). The sandy and ooidal Bad Heart Formation was shown to be restricted to the plains, where it onlaps westward onto the top of the Marshybank Member and pinches out before reaching outcrop (Donaldson et al., 1998, 1999). Subsequent study of additional outcrop and surface sections in the Peace River Plains, coupled with dinoflagellate biostratigraphy, confirmed the existence and significance of the erosional contact between the Bad Heart Formation and the underlying Muskiki Formation, or older strata (Kafle et al., 2013).

Within the Muskiki Member, Plint (1990) traced two regional flooding surfaces, informally termed M1 and M2 (equivalent to surfaces CS4 and CS15 in the present study), and also divided the Marshybank Member into 12 informal “units, termed A through L, bounded by erosion surfaces. Simple upward-coarsening “parasequences” in the offshore part of the basin could be traced westward (landward) to areas where the rocks became sandier and commonly included clean, well-sorted, hummocky- and swaley-cross-stratified sandstone deposited in a shoreface environment (Plint, 1991; Plint and Norris, 1991). Where exposed in the Rocky Mountain Foothills, most shoreface sandstone bodies have a sharp, erosional basal contact that was interpreted as evidence for deposition during relative sea-level fall (Plint, 1991; Plint and Norris, 1991). Thus it could be shown that upwardshoaling “parasequences” deposited offshore, passed laterally landward into “sequences” that preserved evidence for both relative sea-level rise and fall. The sharp-based shoreface sandbodies in the Marshybank Member provided some of the initial stimulus to develop the concept of “forced regression” (Plint, 1991), and ultimately, contributed to the establishment of the falling-stage systems tract (Plint and Nummedal, 2000), which is now the widely accepted “fourth component” of the standard depositional sequence model (e.g., Catuneanu, 2006).

A reconnaissance study by Durbano (2009), extended allostratigraphic correlations southward, enabling Grifi (2012) to establish an allostratigraphic scheme for Coniacian rocks over 60,000 km² of southern Alberta, extending from Township 26 southward into the most northerly Montana Plains (fig. 3, inserted after p. 52). Grifi (2012) extended correlations from the subsurface of Alberta and Montana to outcrop sections, including the Type Section of the Kevin Member of the Marias River Formation, and to various sections in the Alberta Foothills. It was shown that depositional sequences in the lower (Coniacian) part of the Kevin Member in the Type Section near Kevin, MT, could be correlated, via wireline logs, to the Muskiki and Marshybank rocks in Canada (Grifi et al., 2013). Current investigation (Hooper, in prep.) has now linked the correlations of Plint (1990) and Grifi (2012) in the area between townships 26 and 44 (fig. 3).

Data and Methods

The results reported here are founded on a series of linked studies that, collectively, involved over 4800 wireline well logs, 74 outcrop sections, and 30 cores, spanning about 200,000 km² of the foredeep in Alberta and northern Montana (Norris, 1989; Plint, 1990; Plint and Norris, 1991; Donaldson, 1997; Donaldson et al., 1998, 1999; Durbano, 2009; Grifi, 2012; Grifi et al., 2013; Hooper, Ph.D. in prep.; fig. 3, after p. 52). Allostratigraphic units were defined by marine flooding surfaces, recognizable in wireline logs by an abrupt increase in gamma-ray intensity and a decrease in resistivity, corresponding to an increase in clay content. These surfaces were traced and “looped” through grids of log cross sections to ensure that they were correlated consistently (figs. 4–13, inserted after p. 52). Outcrop sections in the foothills were palinspastically restored using balanced structural cross sections constructed by Rottenfusser et al. (2002) on the basis of outcrop mapping, drilling, and seismic data. On the base map (fig. 3), outcrop sections relevant to the present study are shown in their present and restored positions, and distances between wells and outcrop are noted on the cross sections (figs. 4–13, after p. 52).

Most major outcrop sections of Coniacian strata exposed in the Rocky Mountain Foothills were measured in detail, with particular attention paid to depositional cyclicity and bounding surfaces that were indicative of relative sea-level changes (appendix 1). A gamma-ray log was made for selected outcrop sections using an Exploranium GR-130 portable gamma-ray spectrometer, with a sampling period of 30 seconds per site. Simultaneous collection of in situ molluscan macrofauna was conducted, with specimens located precisely in each measured section.

All fossil material collected during this investigation is archived at the Royal Tyrrell Museum of Palaeontology. Details of specimens and accession numbers are given in Walaszczyk et al. and Landman et al., in this issue.

Allostratigraphic Terminology

In order to make an allostratigraphic description of the Muskiki and Marshybank strata, it is necessary to give names to both the stratigraphic surfaces that bound rock units as well as to the rock units themselves. In the following discussion, the regionally mappable flooding surfaces within Coniacian rocks have been designated as CS1 (Coniacian surface 1) to CS23. The erosion surface separating the Cardium Formation from the overlying Muskiki Member is termed E7 following Plint et al. (1986), and the basal surface of Santonian strata is designated SS0 (Santonian surface 0). Allostratigraphically defined rock units are designated CA1 (Coniacian allomember 1) to CA24. Note that allomembers defined herein are considered to be informal stratigraphic units, designated by letters and numbers rather than formally defined names, and therefore are not capitalized in the text (NACSN, 2005).

Carbon-Isotope Analysis

Seven sections through Coniacian rocks (six outcrop, one core) were selected for carbon-isotopic analysis. Results of carbon-isotope analysis for the Cutpick Creek section (fig. 4) are reported here (appendix 2). At this site, 106 samples were collected at 1 m intervals from all lithologies except clean sandstone, in which the organic matter content was too low. Analyses were undertaken at the Stable Isotope Biogeochemistry Laboratory at the University of Durham. Rock samples were ground to a fine powder (ca. 10 μm) using a Retsch agate mortar grinder RM100. The bulk rock powders (ca. 5 mL) were decalcified overnight at room temperature (20° C) using a 3 M HCl solution in 50 mL centrifuge tubes. Insoluble residues were rinsed three or four times in deionized water, dried at 50° C, reground in an agate mortar, and stored in glass vials.

Organic carbon isotope (δ¹³C_org) measurements were performed on 2.5–3 mg splits of the insoluble residues using a Costech elemental analyser (ESC 4010) connected to a Thermo Scientific Delta V Advantage isotope ratio mass spectrometer via a Conflo III interface. Carbon isotope ratios are corrected for ¹⁷O contribution and reported in standard delta (δ) notation in per mil (‰) relative to VPDB. Data accuracy was monitored through analyses of international and in-house standards calibrated against the international standards (viz., IAEA 600, USGS 24, and USGS 40). Analytical uncertainty for carbon isotope measurements was ±0.1‰ for replicate analyses of standards and <0.2‰ on replicate sample analyses.

Establishing Regional Allostratigraphic Correlations

A series of cross sections (figs. 4–13) that integrate wireline logs with outcrop sections, form the physical basis for the new allo- and biostratigraphic scheme presented here. To constrain the stratigraphic relationships between separate outcrop sections scattered along the Rocky Mountain Foothills, a summary, strikeoriented (NW—SE) well-log cross section was constructed (fig. 4), extending more than 750 km from Cutpick Creek, NW Alberta (in the vicinity of Grande Cache; Township 58, latitude 54° 5′ N), to Kevin, Montana (latitude 49° 45′ N; fig. 3). The correlations shown in the strike section are tightly constrained by a large grid of cross sections to the north and east, which are not shown on the base map (Plint, 1990; Grifi, 2012, Hooper, Ph.D. in prep.; fig. 3). Each principal section exposed in the Foothills between Cutpick Creek in the north, and Kevin in the south was correlated to this master cross section using wireline logs, some of which were from wells drilled in the deformed belt (figs. 3, 5–13).

Systematic tracing of marine flooding surfaces through the regional correlation grids of Plint, (1990), Grifi (2012) and Hooper (in prep.), showed that 24 flooding surfaces, designated CS1 to CS23 and SS0, were particularly robust, and could be traced through large parts, if not all of the study area (figs. 4–13).

The Character and Significance of Flooding Surfaces

Although each of the studied allomembers is typified by one or more, upward-coarsening successions (e.g., fig. 14), the nature of the flooding surface that bounds each allomember is variable. Four categories of surface are recognized: either the surface lacks a coarse-grained lag, or one of three types of granule to pebble lag is present. It is important to appreciate that these lags, which may be up to about 20 cm thick, can also be very discontinuous across a given exposure, to the extent that, on a lateral scale of a few meters, coarse grains may be entirely absent from some parts of the flooding surface (fig. 15). In some places, it is clear that lag deposits are confined to shallow erosional depressions on the flooding surface (fig. 16).

No coarse-grained lag: Flooding surfaces CS2, CS3, CS7, and CS9 have no coarse-grained lag and are characterized by an abrupt upward transition from coarser- to finer-grained sediment at which there is no evidence for concentration or winnowing of coarser grains, or formation of an irregular erosion surface. This type of flooding surface provides no evidence of relative sea-level fall and concomitant sea-floor erosion, and therefore is parsimoniously interpreted to record only relative sea-level rise and shoreline transgression, accompanied by a decrease in the volume and caliber of sediment supplied to the shelf.

Lag of intrabasinal clasts: Flooding surfaces CS6 and CS14 locally have a lag comprising intrabasinal phosphate and/or siderite pebbles that range in diameter up to about 100 mm. These clasts are indicative of some degree of erosion, probably to a depth of at least several dm, sufficient to expose early diagenetic nodules and to allow their concentration on the erosion surface. The presence of reworked intraclasts suggests that a phase of relative sea-level fall terminated deposition of the underlying allomember. The absence of extrabasinal pebbles suggests that erosion took place subaqueously, although subaerial emergence can not be ruled out. Subsequent relative sea-level rise reduced current energy at the bed, allowing the clasts to be buried in mud.

Lag of mixed extra- and intrabasinal clasts: In foothills exposures, flooding surfaces E7 and CS1, CS4, CS8, CS11, CS13 and CS15 to CS23 inclusive are, at least locally, mantled with a mixture of extrabasinal chert pebbles, typically 5–15 mm in diameter, and intrabasinal phosphate and siderite pebbles, typically 20 to 100 mm in diameter. Because of the asymmetry of wave-generated currents in shallow water, pebbles are not transported seaward across a marine shelf for more than a few hundred m to a few km from shore (e.g., Clifton, 2006), and it is not possible to concentrate chert pebbles by winnowing underlying sediment lacking such clasts. It is therefore concluded that the presence of chert pebbles on a flooding surface is an indication that the surface had previously been subaerially exposed, when extrabasinal pebbles were supplied by rivers flowing to a lowstand shoreline. This interpretation was initially advanced to explain the extensive mantles of chert pebbles on erosion surfaces in the Cardium Formation (e.g., Plint et al., 1986). Applying similar reasoning, it is inferred that at least the more westerly parts of the chert pebble-bearing surfaces in the Muskiki and Marshybank members experienced a phase of subaerial exposure.

FIG. 14.

A portion of the Muskiki Member exposed in Sheep River (fig. 11), illustrating typical sandier-upward successions (arrows) bounded by flooding surfaces. The regionally mappable surface CS3 is indicated.

Lag of mixed extra- and intrabasinal clasts in a matrix of ferruginous clay ooids: In the southern portion of the study area, flooding surface CS23 is overlain by 0.1 to 1.2 m of ooidal ironstone, the base of which is sharp but typically intensely burrowed by a Glossifungites ichnofauna (fig. 17). Well-rounded chert pebbles, typically 5–15 mm in diameter, and somewhat larger phosphate pebbles are distributed throughout the ooidal ironstone, which lacks stratification and appears to be highly bioturbated. Ooidal ironstone is commonly interpreted to form under conditions of protracted clastic sediment starvation, accompanied by repeated winnowing and erosion of the sea floor in shallow water (e.g., Donaldson et al., 1999, and references therein). Regional correlation shows that the ooidal ironstone, which forms a patchy blanket across southern Alberta and northern Montana, is spatially coincident with a region of subtle stratal upwarp, across which Upper Coniacian strata are either very thin or absent (Grifi, 2012; figs. 4, 9–11). Stratigraphic relationships therefore suggest that, for much of late Coniacian time, the region blanketed by ooidal ironstone was either subaerially emergent, or so shallowly submerged that no clastic sediment could be accommodated, leading to the formation of a regional unconformity. Deposition of ooidal ironstone above the unconformity is interpreted to have taken place in shallow water across a clastic-starved ramp during the early stage of relative sea-level rise in latest Coniacian time. Toward the northwest, the ooidal ironstone appears to be stratigraphically equivalent to up to 20 m of sandy siltstone that forms allomember CA24; this wedge-shaped allomember accumulated in an area of active flexural subsidence.

FIG. 15.

A. Overview of surface CS1 exposed at river level at the mouth of Oldfort Creek (fig. 10). At right, a coarse-grained wave ripple composed of granules and small pebbles of chert and quartz rests sharply on regional erosion surface CS1. Traced laterally over only 3 m, the coarse lag pinches out and surface CS1 is manifest simply as a sharp mud-on-mud contact; scale bar = 20 cm. B. Detail of the coarse-grained wave ripple.

FIG. 16.

Intraclastic pebble lag resting on surface SS0 at Thistle Creek west (fig. 5). A. General view of siderite pebbles seen from above; scale bar in each image = 20 cm. B. Detail of panel A showing pitted, bioeroded surface of a siderite pebble. C. Side view of conglomerate showing clasts filling a shallow erosional trough (arrows).

Use of Distinctive Flooding Surfaces to Establish Correlation to Outcrop

In order to establish, on a regional scale, the stratigraphic distribution of molluscan faunas collected at outcrop, it was necessary to trace flooding surfaces westward from the master strike line (fig. 4), via well logs, to studied outcrop sections in the Rocky Mountain fold-andthrust belt. For most sections, correlation to subsurface was relatively straightforward, based on matching upward-coarsening depositional successions, a process greatly facilitated by the outcrop gamma-ray log that provided an objective measure of lithology. It is emphasized that the process of outcrop to subsurface correlation was based, as far as was possible, on matching the scale and lithological character of depositional successions, and certain thick bentonites seen at outcrop, with those represented in the nearest wireline well logs; wherever possible, the distribution of molluscan fauna was not used as a basis for correlation. This approach was intended to provide an independent check on the relationship between depositional cyclicity and the stratigraphic distribution of molluscan fossils. A number of distinctive physical stratigraphic markers helped to constrain correlations between outcrop and well log, as discussed below.

E7 surface. The E7 surface separates the Cardium Formation from the overlying Muskiki Member, and constitutes a prominent and robust stratigraphic marker across the basin. In the vicinity of the foothills and in the adjacent subsurface, E7 has up to several tens of meters of erosional relief in the form of NW—SE elongate, NE-facing “steps” (e.g., Wadsworth and Walker, 1991). In consequence, the thickness of rock between E7 and overlying surfaces CS1 and CS2 can vary dramatically, reflecting the progressive onlap and burial of preexisting topography by lower Muskiki sediments. The E7 surface has a ubiquitous mantle of extrabasinal chert and quartz pebbles, typically a few cm to a few dm thick.

Surface CS1. Surface CS1 is a robust log marker that caps a distinct, upward-coarsening succession of mudstone, siltstone, and very finegrained sandstone. In most outcrop sections, CS1 is mantled by a few mm to a few cm of anomalously coarse-grained sediment that may range from medium-grained sand to granules or small siliceous pebbles that may be molded into large-scale wave ripples (fig. 15). This thin but distinctive, coarser-grained lag forms a consistent stratigraphic marker. In some sites (e.g., Oldfort Creek, fig. 10), the granule lag on CS1 was discontinuous across an exposure tens of meters wide (fig. 15), raising the possibility that the apparent absence of the lag at some sites (e.g., Blackstone River, fig. 6, Highwood River and Sullivan Creek, fig. 12) may be attributed simply to limited lateral exposure.

Surface CS4. Over much of the study area, surface CS4 caps a prominent, sandier upward heterolithic succession, typically 10–20 m thick, comprising cm-scale interbeds of wave-rippled, very fine-grained sandstone and mudstone in which the bioturbation index (BI) is very low (fig. 18A). In the more westerly foothills exposures, such as Bighorn Dam, Bighorn River, Wapiabi Creek, and Sheep River (fig. 18B, C), CS4 is mantled by a mm to cm-scale lag of small phosphate and chert pebbles typically <5 mm in diameter, whereas in more eastern sections, this lag is absent.

Surface CS11. In the north, at Cutpick Creek (fig. 4) allomember CA11 is mantled by a few cm of chert and intraclastic pebbles dispersed through a sideritized mudstone matrix. Allomember CA11 has a distinctive log signature that can be traced southward in well logs (fig. 4). However, the upper surface, CS11 does not carry a pebble lag in the more southerly outcrop sections described here. Instead, at Chungo Creek, Bighorn Dam, Sullivan Creek, and Highwood River, CS11 is mantled by a sharp-based, cmscale bed of clean, fine- to medium-grained sandstone that forms hummocky cross stratification (HCS) or combined-flow ripples, or fills gutter casts. This thin bed suggests some degree of storm-winnowing and erosion of the top of allomember CA11 that could be an expression of relative sea-level fall.

Surface CS14. Throughout the central foothills, between Thistle Creek (fig. 5) and Mill Creek (fig. 13), surface CS14 marks an abrupt facies boundary between underlying, cm-scale interstratified very fine-grained sandstone and mudstone, and overlying, intensely bioturbated siltstone and silty sandstone that characterizes the lithostratigraphic Marshybank Member (e.g., Stott, 1963; fig. 19A). In wireline logs, particularly toward the east, CS14 is manifest as a distinct “hot” spike on the gamma ray log, suggestive of an enrichment in clay and/or organic matter. Locally, as at Cardinal River (fig. 5), Burnt Timber Creek (fig. 9), and Sheep River (fig. 11), CS14 is mantled by a thin lag of siderite and phosphate pebbles. Toward the north, CS14 is truncated by CS15 (fig. 4).

Surface CS15. In the north at Cutpick Creek, CS15 is mantled by chert pebbles and lithic intraclasts. From north to south, CS15 progressively truncates allomembers CA11, 12, 13, 14, and 15 (fig. 4). Surface CS15 persists southward as a distinctive flooding surface until Township 36, where it is truncated by surface CS23. Traced westward, CS15 is a subtle flooding surface, locally mantled with chert and/or intraclastic pebbles, seen at Bighorn Dam and Bighorn River (fig. 7), Burnt Timber Creek (fig. 9), and Sullivan Creek (fig. 12).

Surface CS16. This surface caps a very widespread sandier-upward succession that, in the north, includes dm-scale sandstone beds with HCS, (e.g., Cutpick Creek, fig. 4). Within allomember CA16, stratified sandstone in the north grades laterally into highly bioturbated silty sandstone toward the south (e.g., figs. 5, 6, 7). Surface CS16 is mantled in the north by chert pebbles whereas to the south, pebbles are absent and the surface may be marked by a subtle flooding surface, or simply a band of large siderite nodules.

Surfaces CS17 to CS22. Surfaces CS17 to 22 are either mantled with a veneer of chert pebbles, or have an irregular, eroded upper surface and/or deep burrows, including Thalassinoides, suggestive of a firm-ground and early lithification. This suite of surfaces is here represented only at Cutpick Creek (fig. 4), but most of these surfaces have been traced for at least 150 km further to the NW of the present study area (Plint, 1990; Plint and Norris, 1991).

Surface CS23. An overall upward-coarsening succession is evident in the lithostratigraphic Marshybank Member (i.e., those rocks above CS14). The upper boundary of that regressive succession (i.e., the maximum regressive surface) is surface CS23 (fig. 20). This is a regionalscale surface that has been traced for 150 km to the north of the present study area (Plint, 1990), and for at least 300 km into Montana to the south of the study area (e.g., Cobban at al., 2005), for a minimum N—S extent of at least 1200 km (fig. 21). Surface CS23 is typically mantled by a lag of chert pebbles and/or intraclasts (phosphate, siderite) that form a layer that may range from a layer only one clast thick to a layer several dm (and exceptionally a few m) thick. South of about Township 26, and extending at least 170 km east to Range 13 W4, CS23 is overlain by 0.1 to 1.2 m of ooidal ironstone containing scattered small chert and phosphate pebbles, and typically pervasively cemented by siderite (Grifi, 2012; fig. 17). The basal surface of the ooidal ironstone is sharp, erosive, and typically highly burrowed (fig. 17). This pebbly ooidal ironstone is correlative with the MacGowan Concretionary Bed mapped over a large part of Montana by Cobban et al. (2005; fig. 4).

FIG. 17.

A. Ooidal ironstone containing small chert pebbles overlying a heavily bioeroded surface near the top of the Coniacian succession on Sullivan Creek (fig. 12). The base of the ironstone is flooding surface CS23. Flooding surface CS15 is mantled with cm scale rounded siderite and phosphate pebbles and surface CS14 marks the boundary between underlying, well-stratified sandstone and mudstone and overlying, highly bioturbated sandy siltstone. B. Detail of bioerosion on the base of the ooidal ironstone shown in (A). Arrows indicate cm scale Thalassinoides, whereas larger irregular erosional structures may also be attributed to Thalassinoides, but may be the result of burrowing by larger arthropods and are perhaps comparable to the “subway tunnels” described from the Cardium Formation (Pemberton et al., 1984); scale bar = 20 cm. C. Detail of the ooidal ironstone above surface CS23 at Mill Creek (fig. 13). A dense network of large Thalassinoides (arrows) penetrate up to 30 cm into the underlying sandy siltstone; hammer is 28 cm long.

Surface SS0. Above CS23, bioturbated silty sandstone and siltstone form allomember CA24, which is an eastward-thinning unit up to about 20 m thick (e.g., fig. 8). CA24 comprises an overall upward-fining stack of meter-scale, upwardcoarsening successions bounded by flooding surfaces that commonly are mantled by a mixture of chert, siderite and phosphate pebbles (e.g., fig. 16). Successive upward-coarsening successions show a gradual upward decrease in BI, with progressively better preservation of cmscale primary stratification. The top of this relatively coarse-grained package is marked by a sharp or abruptly gradational (few cm) transition, at surface SS0, into dark grey, thinly bedded mudstone with a BI of 0–1, characterized by a much higher radioactivity than the underlying rock (fig. 19B). SS0 is particularly prominent in the western part of the study area where the contrast between under- and overlying lithologies is greatest; SS0 gradually becomes more subtle toward the east as allomember CA24 thins. Toward the north, SS0 gradually onlaps onto surface CS23 such that the two surfaces are indistinguishable at the scale of well logs (fig. 4). Where exposed at Cutpick Creek, CS23 lies at the base of a 1 m thick conglomerate capping the Marshybank Member, and SS0 may lie at the same surface, or possibly may be unrecognizable within, or at the top of the conglomerate.

FIG. 18.

A. Overview of the lower part of the Coniacian succession at Burnt Timber Creek (fig. 9) illustrating depositional cyclicity and bounding surfaces E7 and CS1 to CS5. Note the prominent sandier-upward succession culminating in surface CS4. B. Overview of part of the Sheep River canyon (fig. 11). The CS4 surface caps an upward-coarsening package that, in this view, is repeated by a thrust fault; Walaszczyk (circled) provides an indication of scale. C. Detailed view of the subtle, but sharp CS4 surface that is mantled with a few mm of chert and phosphate pebbles and granules and separates bioturbated siltstone and very fine-grained sandstone below from silty clay above.

Continuity of Bounding Surfaces

Figures 4 to13 show that most of the flooding surfaces that bound allomembers can be traced as nearly parallel surfaces for at least 750 km along the strike of the proximal foredeep, and for many tens of km westward to outcrop. Locally, however, some allomembers do lap out. Allomembers CA1 and CA2 onlap locally against erosional topography on the underlying E7 surface (e.g., fig. 4 between wells 3-16-53-25W5 and 10-30-54-25W5). Allomembers CA3 to CA14 form a near-tabular set of strata, traceable throughout most of the strike section. However, allomembers CA5 and CA9 lap out northward whereas CA6 and CA8 lap out southward (fig. 4). Allomembers CA16 to CA23 are confined to a northern depocenter (fig. 4), and extend beyond the northern limit of the present study area. To the east of the proximal foredeep, a number of disconformities have been recognized, across which various allomembers lap out, or are truncated (Hooper and Plint, 2016; Hooper, Ph.D. in prep.). These stratal terminations take place across narrow linear regions that might reflect differential subsidence and uplift across underlying structures (cf. Grifi et al., 2013); discussion of this aspect of the stratigraphy is beyond the scope of the present study.

Principal Molluscan Zones

Although physical allostratigraphic correlation provides a good basis for understanding regional stratigraphy, confidence in correlation is greatly strengthened by integration of key elements of the molluscan fauna. Five key biostratigraphic events have been identified based on inoceramids (fig. 24; see Walaszczyk et al., this issue, for detailed discussion):

FIG. 20.

A. The upper part of the Marshybank Member at the Bighorn Dam section (fig. 7). Surface CS23, which is mantled with chert and siderite pebbles, marks a maximum regressive surface and is overlain by four subtle upward-coarsening successions culminating in surface SS0. B. Upper part of the Marshybank Member exposed on the Bighorn River (fig. 7) showing bioturbated sandy siltstone grading up into weakly stratified silty sandstone, the top of which is marked by surface CS23.

Four key biostratigraphic events have also been identified based on scaphitid ammonites (Landman et al., this issue):

Carbon-Isotope Stratigraphy

Regional stratigraphic correlation (fig. 4) shows that the Cutpick Creek section contains six disconformities at surfaces E7, CS4, CS8, CS11, CS15, and CS23. In representing the carbon-isotope stratigraphy for this section, it is necessary to accommodate these hiatuses. The stratigraphic log, as well as the corresponding carbon-isotope curve, is therefore presented in an “expanded” form to include gaps that are approximately proportional to the thickness of the missing parts of the section (fig. 25). Tie-points between Alberta and the UK Chalk succession (Jarvis et al., 2006), were established on the basis of: (1) The lowest occurrence of Cremnoceramus crassus crassus immediately above the E7 surface and (2) The lowest occurrence of Volviceramus koeneni immediately above surface CS4. The lowest occurrence of Clioscaphites saxitonianus was inferred, based on correlations in figures 4–13, to be at or immediately above surface SS0. Unfortunately, neither scaphitid ammonites, nor Sphenoceramus subcardissoides or S. pachti are present in the UK Chalk succession and hence can not be used as biostratigraphic tie points.

The carbon-isotopic events recorded at Cutpick Creek are tentatively correlated to the English Chalk Reference curve (fig. 25). Greater confidence is placed in the correlation of the lower and lowermiddle Coniacian strata where biostratigraphic control is good. The Light Point, East Cliff, and White Fall carbon-isotope events (CIE) appear to be recognizable with some degree of confidence. The carbon-isotope record at Cutpick Creek is also tentatively correlated with the Coniacian record from hemipelagic sediments sampled in the USGS Portland core in central Colorado (Joo and Sageman, 2014). Biostratigraphic control is provided by the lowest occurrences of Volviceramus and Scaphites (S.) depressus, allowing tentative correlation of the Light Point, East Cliff, and White Fall CIE.

Although a carbon-isotope curve was published for the upper Albian to Santonian succession in a core sampled at Cold Lake in eastern Alberta (Schröder-Adams et al., 2012), the location of that core, on the crest of the forebulge, has resulted in a very fragmentary stratigraphic record. The Lower Coniacian is not represented and the middle and upper Coniacian isotopic record has neither sufficient character nor biostratigraphic control to establish a correlation with the Cutpick Creek section.

Tectonic Control on Deposition

Grifi et al. (2013) showed that, in southern Alberta, Coniacian rocks could be grouped into three distinct packages that had been deposited in three discrete depocenters. Allomember CA1 (equivalent to rocks between surfaces E7 and ME1 of Grifi et al., 2013) occupies an arcuate, westward-thickening basin centered at approximately Township 25, whereas allomembers CA2–CA10 (equivalent to rocks between surfaces ME1 and ME7 of Grifi et al., 2013) collectively occupy a SW-thickening basin with a center located south of the Alberta—Montana border. The abrupt alongstrike shift in the locus of flexural subsidence was attributed to a corresponding shift in the region of active tectonic thickening in the adjacent orogenic wedge. Allomembers CA12–CA15 (equivalent to rocks between surfaces ME7 and DE1 of Grifi et al., 2013) form a thin tabular sheet over the present study area, but thicken markedly to the east where they fill an elongate trough interpreted to have subsided as result of forced folding above an actively extending normal fault in the Precambrian basement (Grifi et al., 2013).

The patterns of subsidence recognized by Grifi et al. (2013) in the south can be traced northward through the present study area. The larger perspective afforded by the present study, coupled with geochronological control (figs. 25, 26), shows that lower Coniacian rocks (allomembers CA1–CA4) occupy a broadly saucer-shaped, SW-thickening basin that thins toward both the NW and SE; this depocenter appears to have been active for about 500,000 yr. Middle Coniacian rocks (allomembers CA5–CA14) occupy a SW-thickening basin that underwent active subsidence for about 1.5 m.y. Toward the NW part of that basin, allomembers CA11–CA14 are progressively truncated by surface CS15 (fig. 4). Upper Coniacian rocks, constituting allomembers CA15–CA23, occupy a distinct northern basin that subsided for about 1 m.y; Upper Coniacian strata are truncated toward the SE by surface CS23. It is clear, therefore, that during the Coniacian the location of the active flexural depocenter shifted episodically along strike by several hundred km. Each actively subsiding basin was rimmed by a corresponding peripheral region of subtle upwarp and stratal truncation. This pattern of shifting depocenters is directly analogous to that mapped in overlying Santonian—Campanian strata (Hu and Plint, 2009; Plint et al., 2012a). The episodic subsidence of discrete, arcuate depocenters may have been a response to the development of localized salients in the adjacent deformed belt. Such salients may have developed in response to a locally thicker sedimentary cover succession, a locally weaker detachment, or a localized indentor, amongst other reasons (cf. Macedo and Marshak, 1999).

FIG. 22.

Offshore facies typical of the Muskiki Member. A. Weakly bioturbated (BI 0-1) cm-scale beds of very fine-grained sandstone interstratified with mudstone forming an upward-coarsening succession culminating at flooding surface CS4. The rusty-weathering colour is typical of this facies and is due to abundant disseminated pyrite. Overlying rock comprises weakly bioturbated mudstone with mm-scale siltstone interbeds. Example: “Brown Creek” section (fig. 6, 11–19 m). B. Stratified mudstone (BI 1-2) with mm-scale coarse siltstone interbeds and abundant, dispersed siderite nodules; scale bar = 20 cm. Example: Oldfort Creek (fig. 10, 42–46 m). C. Heavily bioturbated (BI 4-5) sandy siltstone with dispersed siderite nodules. Example: Oldfort Creek (fig. 10, 49–52 m).

FIG. 23.

Near-shore, sandstone-rich facies in the Marshybank Member in Cutpick Creek (fig. 4, 90–120 m). Bounding surfaces CS18 to CS23 and allomembers CA18 to CA23 are indicated. The upper part of CA18 contains fine-grained sandstone with hummocky cross stratification and the lower part of CA19 contains meter-scale, sandstone-filled gutter casts. The upper part of CA19 and most of CA20 consists of well-sorted, swaley cross-stratified, fine-grained sandstone. Centimeter-scale fine sandstone beds in CA21 to CA23 are planar laminated and wave rippled.

FIG. 24.

Summary chart showing the stratigraphic distribution of fossils within the 24 allomembers mapped through the study area.

FIG. 24. Continued

Summary chart showing the stratigraphic distribution of fossils within the 24 allomembers mapped through the study area.

FIG. 24. Continued

Summary chart showing the stratigraphic distribution of fossils within the 24 allomembers mapped through the study area.

FIG. 24. Continued

Summary chart showing the stratigraphic distribution of fossils within the 24 allomembers mapped through the study area.

FIG. 24. Continued

Summary chart showing the stratigraphic distribution of fossils within the 24 allomembers mapped through the study area.

FIG. 24. Continued

Summary chart showing the stratigraphic distribution of fossils within the 24 allomembers mapped through the study area.

FIG. 25.

Comparison of the organic carbon isotope stratigraphy at Cutpick Creek with the carbonate carbon isotope reference curve for the English Chalk succession (after Jarvis et al., 2006, recalibrated to GTS2012 after Laurin et al., 2015). The Cutpick section, and corresponding carbon-isotope curve, has been “expanded” to show the location of hiatuses determined on the basis of regional subsurface correlation (i.e., fig. 4). Biostratigraphic collections made in all the Coniacian sections included in this study (fig. 24), allow three tiepoints to the UK Chalk to be established at the lowest occurrence (LO) of Cremnoceramus crassus crassus, and the lowest occurrence of Volviceramus. The base of the Santonian is defined at the co-appearance level of Clioscaphites saxitonianus and Sphenoceramus ex gr. pachti, which corresponds to surface SS0 of this study. The lowest occurrence of C. crassus, Volviceramus and Scaphites (S.) depressus allows tentative correlation to the Portland core, which does not extend as high as the Coniacian—Santonian boundary (Joo and Sageman, 2014). The highest local occurrence of Inoceramus gibbosus, which is widely distributed in allomembers CA3 and CA4 in the western Alberta foredeep, is marked by the lag-strewn erosion surface CS4, suggestive of significant sea-level fall and subsequent transgression. The absence of the I. ex gr. gibbosus interval in most sections of topmost lower Coniacian strata in other parts of the world suggests that a significant hiatus exists at the base of the overlying Volviceramus Zone.

Origin of Allomembers

The excellent subsurface control available in the study area has allowed 24 Coniacian allomembers to be mapped with confidence. Allomembers are typically <10 m thick, and rarely exceed 20 m (figs. 4–13), yet can be traced along and across strike for hundreds of km. Given the ~3.0 m.y. represented by the studied interval of strata, each of the 24 allomembers can be inferred to have had an average duration of ~125,000 yr. The ubiquitous upwardcoarsening signature of allomembers, coupled with the widespread presence of intra- or extrabasinal pebble lags on flooding surfaces, indicates that many allomembers can be interpreted as depositional sequences, each of which embodies evidence for initial relative sea-level rise, followed by shoaling, and terminated by relative sea-level fall that led to erosion of the sea floor, and in some cases, to subaerial emergence. The lateral continuity, over hundreds of km, coupled with the extremely high aspect-ratio of each allomember suggests that depositional cyclicity was a response to allogenic forcing, rather than to localized autogenic effects such as delta lobe switching. It is possible that depositional cyclicity was the result of changes in the rate of clastic sediment supply that resulted in alternating coastal progradation and transgression, manifest as upward-shoaling successions bounded by flooding surfaces. However, extrabasinal pebble lags imply episodes of relative sea-level fall and subaerial exposure that can not be explained solely in terms of a changing rate of sediment supply.

Relative sea-level rise and fall can be effected by both tectonic and eustatic mechanisms. The stratigraphic data presented herein (figs. 4–13, 26), provide evidence that stratal packages composed of several allomembers form large-scale arcuate wedges that are most reasonably explained in terms of differential flexural subsidence and uplift on length scales of hundreds of kilometers. This pattern contrasts sharply with the relatively tabular geometry of individual allomembers. Given the evidence that tectonic subsidence due to static loading resulted in broad, saucer-shaped depocenters, it seems difficult to also attribute the tabular geometry of allomembers to the same mechanism. Allomembers are therefore most simply explained as a consequence of relatively high-frequency (order of 100,000 yr) eustatic sea-level cycles superimposed on relatively low-frequency (order of 0.5 to 1.5 m.y.), and spatially non-uniform pulses of tectonically driven subsidence.

Nature of Biozonal Boundaries

Important biotic colonization events that mark the lowest occurrences of Cremnoceramus crassus crassus, Volviceramus, Sphenoceramus subcardissoides, and S. pachti, all correspond to widely mappable erosion surfaces that indicate relative sea-level fall followed by transgression. Sea-level changes therefore appear to have played an important role in speciation and extinction events. Because marine flooding surfaces can be considered to have formed geologically instantaneously, it is also reasonable to interpret the boundaries of the inoceramid biozones to closely approximate time planes.

FIG. 26.

Summary representation, to scale, of stratal geometry viewed in strike (NW-SE) and three dip (NESW) sections spanning the study area. Lower Coniacian strata fill a saucer-shaped depocenter that thins to both NW and SE, whereas Middle Coniacian strata are thickest in the south but are erosionally truncated toward the NW. Upper Coniacian strata fill a depocenter in the NW but are truncated toward the SE, and are largely absent over most of southern Alberta and northern Montana. Sections summarized from figures 4, 8, 11, and 13.

The coincidence between the lowest and highest occurrences of scaphitid ammonite species and erosional surfaces subsequently modified by transgression is less clear. The lowest occurrence of Scaphites (S.) preventricosus is just above erosional surface E5.5, which marks the beginning of a major transgression (Walaszczyk et al., 2014). However, the lowest occurrence of S. (S.) ventricosus is immediately above surface CS2, in allomember CA3, close to an interpreted highstand. The lowest occurrence of S. (S.) depressus is just above surface CS14, which marks a high-frequency flooding surface during a long-term shallowing trend that continued, punctuated by minor transgressions, through much of the late Coniacian. In contrast, the lowest occurrence of Clioscaphites saxitonianus is at the base of the Santonian (surface SS0), coinciding with a major transgression and a marked change in facies to deeper-water, more offshore mudstones.