Tectonic, stratigraphic, and sedimentologic ...

Tectonic, stratigraphic, and sedimentologic significance of a regional discontinuity in the upper Judith River Group (Belly River wedge) of southern Alberta, Saskatchewan, and northern Montana1

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Royal Tyrrell Museum of Palaeontology, Box 7500, Drumheller, Alta., Canada TOJ OYO AND

Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, 3303 - 33rd Street NW, Calgary, Alta., Canada T2L 2A7 Received March 3, 1992 Revision accepted September 3, 1992 The lithostratigraphic interval between the Taber and Lethbridge coal zones in the upper portion of the nonmarine Judith River Group of southeastern Alberta is divisible into two lithostratigraphic units separated by a regionally extensive and diachronous discontinuity. The lower unit, referred to here as the Oldman Formation, is characterized by very fine grained to fine-grained sandstones that contain fewer than 2 % volcanic rock fragments; sandstone bodies with numerous sets of horizontally stratified sandstone, showing little or no evidence of lateral accretion; siliceous paleosols (ganisters); and a relatively high gamma-ray signal in the upper half of the formation. The Oldman Formation comprises deposits of a lowsinuosity, perhaps ephemeral fluvial system that originated in the southern Cordillera of Canada and northern Montana and flowed northeastward, perpendicular to the axis of the Alberta Basin. The upper unit is assigned to a new formation, the Dinosaur Park Formation, and is characterized by fine- to mediumgrained sandstones with up to 10% volcanic rock fragments; sandstone bodies that exhibit lateral-accretion surfaces in the form of inclined heterolithic stratification; numerous articulated dinosaurs and dinosaur bone beds; and a relatively low gamma-ray signal in the lower half of the formation. The Dinosaur Park Formation comprises deposits of a high-sinuosity, fluvial-to-estuarine system that originated in the north and central Cordillera and flowed southeastward, subparallel to the axis of the Alberta Basin. 40Ar/39Arand K-Ar dating of Judith River Group bentonites shows that the contact between the Oldman and Dinosaur Park formations becomes younger toward the south and southeast. These data also demonstrate that the Dinosaur Park Formation clastics migrated southeastward at a rate of approximately 130- 140 km/Ma, gradually overstepping the Oldman Formation clastics. The widely recognized north-to-south increase in intensity of overthrust loading along the western margin of the Alberta Basin during the Late Cretaceous is thought to be responsible for (i) differences in accommodation space for the proximal portions of the Oldman and Dinosaur Park formations, and (ii) the establishment of a southerly tilt in the Alberta Basin leading to the southeastward migration of the Dinosaur Park Formation clastics. In the northern portion of the basin, relatively lower rates of subsidence, combined with periods of isostatic rebound in the foredeep, resulted in the southeastward migration of Dinosaur Park Formation clastics as sediment input exceeded accommodation space. In the southern portion of the basin, relatively higher rates of subsidence and little isostatic rebound acted to trap coarse-grained Oldman Formation clastics in the foredeep and may have led to periods of sediment starvation in more distal portions of the basin. An inferred lower depositional slope associated with the Dinosaur Park Formation (relative to the Oldman Formation) is thought to have resulted from gradual loading of the basin as Dinosaur Park Formation clastics migrated southeastward or some form of tectonically induced subsidence. L'intervalle lithostratigraphique entre les zones houillkres de Taber et de Lethbridge, localist dans la portion suptrieure du Groupe de Judith River non marin, dans le sud-est de I'Alberta, est divisible en deux unitts lithostratigraphiques qui sont sCparCes par une discontinuit6 diachronique d'extension rtgionale. L'unitt inftrieure, dtsignCe ici par la Formation d'Oldman, est caractCrisCe par des grks B grain trbs fin a fin renfermant moins de 2% de fragments de roches volcaniques; des corps de grks incluant de nombreux ensembles de lits d'un grks stratifit horizontalement, ne presentant pas ou peu d'indices d'accrttion latkrale; des paleosols siliceux (sables siliceux purs sous les couches de charbon); et par un signal de rayonnement gamma relativement intense dans la moitit supCrieure de la formation. La Formation d'Oldman comprend des dtp6ts de faible sinuositk, et peut-&tre un systbme fluvial Cphtmkre qui a pris naissance dans le sud de la Cordillbre du Canada et le nord du Montana, et qui s'tcoulait vers le nord-est dans une direction perpendiculaire a I'axe du bassin de 1'Alberta. L'unitC supCrieure est assignCe B une nouvelle formation, la Formation de Dinosaur Park, laquelle est caractCrisCe par des grks B grain fin a moyen pouvant contenir jusqu'i 10% de fragments de roches volcaniques; des corps de grks qui exhibent des surfaces d'accrttion latCrale sous forme de stratification hCtCrolithique inclinte; de nombreux dinosaures articults et des couches de dCbris de dinosaures; et par un signal de rayonnement gamma relativement faible dans la partie inftrieure de la formation. La Formation de Dinosaur Park renferme des dCp6ts caracttrises par une forte sinuositt, et un systkme allant de fluvial a estaurien qui a pris naissance dans le nord et le centre de la Cordillbre, et qui s'tcoulait vers le sud-est dans une direction subparallkle a l'axe du bassin de 1'Alberta. Les datations par 4 0 ~ r / 3 9et~K-Ar r des bentonites du Groupe de Judith River rtvklene que le contact entre les formations dlOldman et de Dinosaur Park devient de plus en plus jeune vers le sud et le sud-est. Ces donntes dCmontrent en plus que les matCriaux clastiques de la Formation de Dinosaur Park ont migrC vers le sud-est B un taux d'environ 130 - 140 km/Ma, Can. J. Earth Sci. 30, 174-200 (1993)

'Geological Survey of Canada Contribution 44391 Printed in Canada 1 lmprtme au Canada


EBERTH AND HAMBLIN

et qu'ils recouvraient graduellement par transgression les materiaux clastiques de la Formation d'oldman. L'accroissement en intensite du nord vers le sud de la charge chevauchante le long de la bordure occidentale du bassin de 1'Alberta durant le CrktacC tardif, largement admis, est interpret6 comme la principale cause (i) des differences de l'espace disponible d'accommodement des portions proximales des formations d'Oldman et de Dinosaur Park, et (ii) du basculement vers le sud subi par le bassin de I'Alberta, ce qui a dirigC vers le sud-est les matCriaux clastiques de la Formation de Dinosaur Park. Dans la partie borCale du bassin, les taux de subsidence qui Ctaient relativement plus faibles, combines avec les pCriodes de rebondissement isostatique dans l'avant-fosse, ont force la migration vers le sud-est des matCriaux clastiques de la Formation de Dinosaur Park lorsque l'apport de sediment a excede l'espace d'accommodement. Dans la partie meridionale du bassin, les taux de subsidence relativement tleves et le faible rebondissement isostatique ont contribuC a piCger les materiaux clastiques grossiers de la Formation d'Oldman dans l'avant-fosse, et ces facteurs sont probablement responsables des pkriodes de sedimentation maigre dans les portions plus distales du bassin. Une pente de dCpBt, presumCment plus faible, associCe 2 la Formation de Dinosaur Park (comparativement 2 celle de la Formation d'oldman) est interprttee comme le resultat de l'augmentation graduelle de la charge dans le bassin lorsque les materiaux clastiques de la Formation de Dinosaur Park migrbrent vers le sud-est, ou bien la conskquence d'une forme quelconque de subsidence induite tectoniquement. [Traduit par la redaction]

Introduction The Judith River (Belly River) clastic wedge reflects a major phase of tectonic uplift in the Canadian Cordillera and subsequent sedimentation in the north - south-trending Alberta foreland basin throughout the Campanian (Cant and Stockmal 1989). As is the case with sedimentary prisms deposited in many foreland basins (Allen et al. 1986), the Judith River wedge displays considerable variation in its composition and thickness on a regional scale. thus potentially reflecting paleogeographic and temporal variations in tectonism, eustasy. subsidence, sediment input, and climate. Documenting the compositional variation within the Judith River wedge and cstablishing the lithostratigraphic and chronostratigraphic relationships of its various subunits are prerequisites to unravelling the history of the Alberta Basin during the Campanian and, ultimately, understanding the factors that produced the basin's heterogenous fill. Our examination of outcrop and subsurface data from the Judith River wedge in the plains of southern Alberta, Saskatchewan, and northern Montana indicates that the stratigraphic interval between the Taber coal zone and the marine Bearpaw Formation includes a previously unrecognized diachronous discontinuity that separates two overlapping lithostratigraphic subunits: a lower, south and southwesterly thickening fluvial unit and an upper, north to northwesterly thickening fluvial to paralic unit. The recognition of two geographically distinct and overlapping clastic subunits within the uppermost portion of the Judith River wedge raises interesting questions about ( i ) regional variations in tectonism in the Canadian Cordillera. (ii) the subsidence and sediment input history of the Alberta Basin during this time, and (iii) the regional paleoenvironmental history of the basin during the middle to late Carnpanian. In this paper, we discuss the nature and significance of the discontinuity in the upper portion of the wedge as it relates to the Campanian age history of the Alberta Basin and Canadian Cordillera. We also revise the stratigraphic nomenclature of the Judith River wedge in the southern Alberta plains and define a new lithostratigraphic unit, the Dinosaur Park Formation, which is bounded at its base by the discontinuity. Stratigraphic nomenclature On the basis of new data presented here, we recognize three major lithostratigraphic subdivisions of Judith River clastic wedge (Fig. 1). In ascending order these are the Foremost, Oldman, and Dinosaur Park formations. The lower two subdivisions are essentially identical to the previously recognized

Foremost and Oldman formations of Russell and Landes (1940). The Dinosaur Park Formation is a new lithostratigraphic unit (Appendix 1). We treat the Judith River clastic wedge as a group, raising the previously recognized Judith River Formation (McLean 1971) to group status (Appendix 1).

Methods Absolute ages presented in this paper are based on recently completed 40Ar/39Arand K-Ar dating of sanidine, plagioclase, and biotite phenocrysts from Judith River Group and Bearpaw Formation bentonites in southern Alberta (Eberth et al. 1992; Eberth and Deino 1992; for detailed methodology see Thomas et al. 1990). Standard thin sections (0.03 mm) were utilized to obtain petrographic data. Specimens were cut and mounted with random orientation. Counts were made using a Swift automated point counter system attached to the stage of an Olympus BH-2 petrographic microscope. Three hundred grain counts per slide were tabulated from 19 slides. Spacing distance ranged from 0.25 to 1.00 mm. Two hundred and thirty-seven paleocurrent measurements were made during this study. Of these, 21 1 (89%) were collected from three-dimensionally exposed meso- to large-scale trough cross-strata associated with paleochannel deposits. The remaining data were collected from channel scours (n = 12; 5 %) and ripple cross-lamination (n = 14; 6%). No paleocurrent data from large-scale lateral-accretion surfaces were included in the paleocurrent analyses and rosettes. Paleocurrent data were grouped according to (i) stratigraphic position (i.e., in either the Oldman Formation or Dinosaur Park Formation), and (ii) geographic location. No attempt was made in this study to subdivide the paleocurrent data on the basis of stratigraphic position within the Oldman and Dinosaur Park formations. One hundred and seventy-five paleocurrent measurements were collected from exposures of the Oldman Formation in Dinosaur Provincial Park, along the South Saskatchewan River north of Medicine Hat and along the Milk River in southeastern and southwestern Alberta. Sixty-two measurements were obtained from exposures of the Dinosaur Park Formation in Dinosaur Provincial Park, along the South Saskatchewan River north of Medicine Hat, ~CarnondCity, Havre. and near Muddv Lake. Saskatchewan. Paleocurrent data collected by ~ o s t e ;et al. (1987) from exposures of the Dinosaur Park Formation in Dinosaur Provincial Park were compared with our data during this study. Analysis of paleocurrent data was carried out following the method of Curray (1956). ---

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AGE POL.

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THIS PAPER

RUSSELLLLANDES McLEAN (1971) (1 9401 Bearpaw Fm. Bearpaw Fm.


Oldman Formation

Formation

Foremost Formation

ZONES Baculites compressus 'Baculites

scotti

Judith River Formation

Baculites asperiformis

80- I

- AN 82 -

AMMONITE

33r

-

Pakowki Formation

Pakowki Formation

Claggett Formation

Milk River Fm.

Milk River Fm.

Eagle Fm.

84 FIG. 1. Stratigraphy of the Judith River Group in the plains of southern Alberta showing the newly proposed threefold division. Chronostratigraphic, polarity chron, and biozonation data are from numerous sources (see text).

lloO1

A a

'

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FIG. 2. Regional geographic distribution of the Judith River Group in outcrop (hachure pattern). Stipple pattern represents geographic distribution of the Two Medicine Formation. Dotted rectangle delineates the study area. From Ross et al. (1955), Green (1970), and Broughton (1980).

Geologic and stratigraphic setting The Judith River Group comprises an eastward-thinning, nonmarine-to-paralic clastic wedge that was deposited along the western margin of the Western Interior Basin during

middle to late Campanian time. As defined in this paper, the group can be traced in outcrop for more than 1000 krn from central Alberta and Saskatchewan into south-central Montana (Fig. 2). Farther south in Wyoming, correlative nonmarine strata are included in the more time-inclusive Mesaverde Formation (e.g ., Lillegraven and McKenna 1986). Figure 3 shows the study area and the location of outcrop sites, wells, and cross sections discussed in this paper. Excellent exposures, up to 100 m thick, of the upper portion of the group are present along the Red Deer River at Dinosaur Provincial Park; good to fair exposures occur along the Oldman, Bow, South Saskatchewan, and Milk rivers in southcentral and southeastern Alberta. North of the Red Deer River, in central Alberta and Saskatchewan, the group is known largely from subsurface data (e.g., Wasser 1988), and only limited exposures are present. In northern Montana, exposures of the group are limited to the Milk River drainage at Havre and along the Missouri and Judith rivers where a complete section of the group is exposed. In terms of regional correlation, the Judith River Group thickens to the west and northwest, passing into the nonmarine and more time-inclusive Belly River, Brazeau, and Wapiti formations, whereas in Montana the group passes westward into thick, continental sediments of the Two Medicine Formation (Lorenz 1981). In general, the lower half of the Judith River Group in southern Alberta records the regressive portion of Kauffman's (1977) T/R 8 or Claggett Cycle, whereas the upper half records the onset of the TIR 9 or Bearpaw Cycle. In sequence stratigraphic terms, the Judith River Group of the southern Alberta plains comprises a number of the third-order cycles within the Upper Zuni A-4 Supercycle, including 4.1, 4.2, 4.3, and the lower half of 4.4 (Haq et al. 1988). In biostratigraphic terms, the Judith River - Bearpaw contact in southern Alberta is correlated with the Baculites compressus ammonite biozone (Caldwell 1982). Eberth et al. (1990) correlate the discontinuity between the Oldman and


EBERTH AND HAMBLIN

FIG. 3. Details of study area shown in Fig. 2. Hachure pattern indicates areas of outcrop examined during this study. Stipple pattern indicates the isopach study area shown in Fig. 17. 0,locations of wells that provided geophysical data; 8 , locations of wells that provided core; a, cities and towns: D, Drumheller; H, Havre; L, Lethbridge; M. Medicine Hat; R, Red Deer; U, Unity. Transect labelled NW -SE is shown in Fig. 16a. Transect labelled SW-NE is shown in Fig. 16b. Transect indicated by short spiked line is shown in Fig. 18. Transects labelled C-C', D-D', and E-E' show the location of the similarly labelled transects of Macdonald et al. (1987). ARC CHI-83, Alberta Research Council core hole 1-83.

Dinosaur Park formations near Muddy Lake, Saskatchewan, with the Baculites scotti ammonite biozone and, by using palynostratigraphic data, infer that the discontinuity in Dinosaur Provincial Park correlates with the base of the B. scotti biozone and the top of the Baculites gregoiyensis biozone (Eberth et al. 1990, their Fig. 5 ) . The base of the Judith River Group in southern Alberta is correlated with the Baculites asperiformis biozone (Lerbekmo 1989). The Judith River Group was deposited across a broad alluvial to coastal plain in fluvial, floodplain, estuarine, swamp, and lagoonal environments (Koster et al. 1987; Macdonald et al. 1987). Detailed local facies descriptions are available in Ogunyomi and Hills (1977), Koster et al. (1987), Wood (1989), Eberth (1990a), and Eberth et al. (1990).

Recognition of the discontinuity between the Oldman and Dinosaur Park formations Surface expression at Dinosaur Provincial Park The regional discontinuity separating the Oldman and Dinosaur Park formations can be traced throughout the Judith River Group in southeast Alberta, west-central Saskatchewan,

and northern Montana near Havre. It is best expressed in the exposures of the Judith River Group along the Red Deer River at Dinosaur Provincial Park (Eberth 1990b) where it occurs approximately 80 m below the contact with the marine Bearpaw Formation. Figure 4 shows six stratigraphic sections measured along a 30 krn stretch of the Red Deer River in and around Dinosaur Provincial Park. In each section, the discontinuity is placed at the base of the first sandstone body exhibiting some or all of the characteristics listed in Table 1 and described below.

Petrology and petrography In general, there is an increase in sandstone grain size from very fine and fine to fine and medium upward across the discontinuity. Extraformational granules, pebbles, and cobbles appear as localized strings and thin lenses for approximately 20 m above the discontinuity in the Dinosaur Park Formation (Fig. 5), particularly in exposures along the Red Deer and South Saskatchewan rivers. These clasts comprise, in decreasing abundance, black and grey chert, metamorphic quartzite, and rare nonquartzite metamorphic fragments. The largest

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FIG.4. Six measured stratigraphic sections from the upper Judith River Group in and around Dinosaur Provincial Park, Alberta. Vertical scales in metres. The sections are on file at the Royal Tyrrell Museum of Palaeontology. IHS, inclined heterolithic stratification. C1, claystone; Si, siltstone; F, M, and C, fine- , medium- , and coarse-grained sandstone.

clasts are quartzites, with long axes up to 21 cm. The most likely sources of the extraformational conglomerates in the Dinosaur Park Formation are the Omineca and Rocky Mountain morphogeological belts of the central Canadian Cordillera (Lerbekmo 1963; Rahmani and Lerbekmo 1975; Mack and Jerzykiewicz 1989; Monger 1989, Fig. 2.12). In thin section, sandstones in the stratigraphic interval from the top of the Taber coal zone to the base of the Lethbridge coal zone group into three distinct stratigraphic intervals (Fig. 6). The Oldman Formation can be divided into two inter-

vals of subequal thickness. Sandstones in the lower interval have a low quartz content, a modest volcanic rock fragment content, and a high plagioclase to K-feldspar ratio relative to the overlying intervals. With the exception of a low content of volcanic rock fragments, these sandstones resemble those in the Dinosaur Park Formation. Sandstones in the upper half of the Oldman Formation are highly distinctive and the most mature, containing a high proportion of quartz grains relative to all other species, high percentages of reworked carbonate, and rare volcanic rock fragments (Fig. 7). Sandstones above

-

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EBERTH AND HAMBLIN

TABLE1.Distinguishing characteristics of the Oldman and Dinosaur Park formations


Characteristic

Oldman Formation

Dinosaur Park Formation

Petrology -petrography Typical sandstone grain size Extraformational clasts Quartz grains Reworked CaCO, grains Volcanic rock fragments Plagioclase - K-feldspar

Very fine to fine Absent Abundant (20-60 %) Common (max. 15%) Rare (max. 2 % ) 1:l

Fine to medium Present Common (20 - 30 %) Rare (max. 5 % ) Common (max. 10%) 2: 1

Sedimentology Fine - coarse member division Sandstone bodies Most common lithofacies Macroforms Fine members Canisters Paleocurrents

Subtle Lenticular Se, Shll, St, Sr Rare: IS Sandstone, siltstone, claystone Common North-northeast

Discrete Multistoried sheets Se, St, Sr Common: IHS, IS Claystone Very rare - absent East-southeast

Paleontology Vertebrate microfossils Dinosaurs and bone beds Invertebrate trace fossils Bioturbation

Common in splay deposits Rare Abundant Abundant

Common in channel lag deposits Abundant Rare Common

Geophysical logs Gamma ray emission

Very high in upper half of formation

Very low in lower half of formation

the discontinuity in the Dinosaur Park Formation are immature and possess low percentages of quartz, high percentages of volcanic rock fragments, and high proportions of chert grains relative to quartz and plagioclase relative to alkali feldspars (K-feldspar) (Fig. 7). Smectite clays are particularly abundant in the matrices of sandstones above the discontinuity. In outcrop, these features result in distinctive differences in weathering and color of the sandstones immediately above and below the discontinuity (Fig. 8). Oldman sandstones crop out as yellow, steep-faced and blocky surfaces. In contrast, Dinosaur Park Formation sandstones are greyer in color and crop out as rounded, highly rilled surfaces. On the whole, the Oldman Formation is pale white to yellow, whereas the Dinosaur Park Formation is grey and brown. Sedirnentology The most easily recognized and consistent features with which to identify the discontinuity in outcrop are sedimentological. In the Oldman Formation, coarse and fine lithofacies comprise thin beds that are complexly interbedded and laterally limited. These beds exhibit a wide range of gradational grain sizes, making it difficult to subdivide facies into distinct fine and coarse members. In contrast, lithofacies of the Dinosaur Park Formation comprise thicker, more laterally extensive beds that can be divided easily into clay-dominated fine members and sand-dominated coarse members (e.g., Koster et al. 1987; Wood 1989) (Fig. 4). Below the discontinuity, sand bodies of the Oldman Formation range from ribbons to sheets and are dominated by horizontal and low-angle-inclined strata (Shll, sensu Miall 1992), trough cross-strata (St), and ripple lamination (Sr), including abundant climbing ripples (Figs. 4, 9). Macroforms consisting of large-scale, inclined stratification (lateral-accretion surfaces) are rare and appear to be limited to paleochannels in the uppermost 10 m of the formation. Above the discontinuity, sandstone bodies of the Dinosaur

FIG.5. (a)Extraformational pebbles and cobbles just above the discontinuity in Dinosaur Provincial Park. (b) Quartzite clast (q). (c) Chert clast (c). The diameter of the lens cap is approx. 6 cm.

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142 1 4 0 - 1 -

I

I

Formation t


I Dinosaur Park

---40

-

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Oldman

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30

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2

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6

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% CaC03

% PLAG I % K-SPAR

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% QTZ I % CHERT

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7 Bearpaw Formatlon 142 140

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-'I---------------.--------------------.--Dlnosaur Park Formallon

FIG.6. ( a ) Point count data (300 counts/sample) from 19 sandstone samples in Alberta Research Council core hole 1-83 (see Fig. 3 for location). Bold numbers and shaded rectangles indicate averages of point count data for three stratigraphic intervals. Note differences in scale for each mineralogic component. (b) Summary of data in a. Numbers and widths of blocks represent averages of point count data for each stratigraphic interval. VRF, volcanic rock fragments; QTZ, quartz; CaCO,, calcium carbonate; PLAG, plagioclase; K-SPAR, potassium feldspar. Park Formation form laterally extensive multistoried sheets with overall fining- and thinning-upward sets of trough crossbedding and macroforms consisting of inclined heterolithic stratification and inclined stratification (IHS and IS sensu Thomas et al. 1987; Wood 1989) (Figs. 4, 9). Major sandstone deposits in the Oldman Formation are interpreted here as the remains of lower sinuosity, relatively shallow fluvial channels (