Print this page
Click to view posters in PDF format.
Poster 1 (5.1 mb) Poster 2 (1.6 mb) Poster 3 (4.1 mb)
Right click on poster to save.
PSTide-Dominated Facies Complex at Southern Terminus of Sunburst Sea, Cretaceous Kootenai Formation, Great Falls, Montana*
By
Robert K. Schwartz1 and Susan M. Vuke2
Search and Discovery Article #50045 (2007)
Posted June 12, 2007
*Adapted from poster presentation at AAPG Rocky Mountain Section Meeting, Billings, Montana, June 11-13, 2006. Because of the large file sizes represented by the posters, this adaptation does not include the layout of the original posters. The first digit of each figure numbers refers to the poster number. Text and figure captions are essentially the same as those on the posters.
1Department of Geology, Allegheny College, Meadville, PA, 16335 ([email protected])
2Montana Bureau of Mines and Geology, Butte, MT, 59701 ([email protected])
The Sunburst Member of the Lower Cretaceous Kootenai Formation in the Great Falls area represents the southern terminus of the earliest marine transgression into the Cretaceous foreland of western Montana. Ripple bundles, mud drapes, reactivation surfaces, rhythmically stacked two-part and neap-spring bundles of mud-sand laminations, flaser bedding, and flat-crested wave ripples document tidal dominance. Brackish to marine conditions are indicated by low ichnofossil diversity and extremely rare ammonoid fossils. A 7-14-m-thick, upward-fining sandstone lithofacies that contains channel- shaped erosion surfaces and stacked subhorizontal to low-angle inclined beds of large-scale trough and tabular cross-stratification represents sand-wave shoal development atop a tidal ravinement surface. The sand-wave lithofacies is overlain by less than 34 m of stacked, NE-SW trending, sandstone- and heterolithic-filled channels. Sedimentary structures within the sinuous channels indicate bank accretion, slumping, rhythmic tractive-slack sedimentation, bipolar flow, and minor wave influence. Toward the eastern and southern basin margin, stacked, tabular, upward-coarsening tidal bar successions and mudstone-dominated lithofacies merge laterally into thin tidal-flat and tidal-creek deposits. Tidal-flat and nonmarine coastal-plain facies regionally cap the entire assemblage. Although the Sunburst overlies the NW-SE-striking Sweetgrass Arch, fluvial paleocurrent and isopach data for the total Kootenai indicate a northward paleoslope and possible paleovalley control of Sunburst deposition along a local, elongate, NE-SW zone that transects the South Arch. Overall, the Sunburst represents southward transgressive-to-highstand systems development in a N-S-striking basin-scale embayment. The irregular coast included tide-dominated shorezone regions and estuaries along paleovalley tracts.
The Kootenai Formation in western Montana has traditionally been considered to be fully non-marine (e.g., Walker, 1974; Mudge and Rice, 1982; Berkhouse 1985). In particular, the quartz-rich Third Kootenai member (Sunburst Sandstone) was originally interpreted to be of fluvial and lacustrine origin (Walker, 1974). However, more recent work by Burden (1984), Hopkins (1985), Vuke (1987) and Farshori and Hopkins (1989) indicate a marine to brackish setting.
|
|
Purpose of the Long-term Study 1. Provide detailed facies information for the Sunburst Sandstone and correlative strata in the Great Falls area. 2. Establish sequence stratigraphy relationships. 3. Provide a link between surface and subsurface physical properties.
Purpose of the Poster To demonstrate that: 1. At least 4 major tide-dominated facies make up the Sunburst Sandstone and equivalent strata. - Sand Wave (estuary shoal-and-channel complex) - Estuary Tidal Channel - Subtidal Sand Bar and Flat - Tidal Flat 2. The southern terminus of earliest marine transgression (Barremian) into the Cretaceous foreland was located just south of the Great Falls area and, the Sunburst represents southward transgressive-to-highstand systems development in a N-S-striking basin-scale "embayment."
Stratigraphy and Facies Distribution (Figures 1-1 – 1-6)
(Figures 1-7 – 1-20)
Properties of Sand Wave Facies (Tidal Shoal Bar and Channel) Overall - A relatively extensive, 5 to 14 m-thick, NE-SW elongate (?), upward-fining quartz sandstone body. - Pinches out to east and south toward basin margin; extends N and W into subsurface. - Disconformable lower contact above: (1) Sunburst estuary mudstone and tidal bedding, (2) non-marine Kk2 coastal plain facies, and (3) fluvial lower Kootenai sandstone. - Overlain by tidal flat, subtidal/estuary mud, and tidal channel facies depending upon location.
Internal Properties - A composite of southwestward-elongate bodies having a channel-shaped base and horizontal to slightly convex upper surface. - Vertical structural sequence: upward thinning, tabular to broadly wedge-shaped, cross-stratified beds ranging from about 2.5 m to 5 cm in thickness. Large- to medium-scale two-dimensional ("tabular") and three-dimensional (trough) cross-stratification typical. - Multiple internal erosion surfaces. - Lithic content typically high along the erosional (ravinement) base of the unit with concentrations of mudstone rip-up clasts. - Plesiosaur bone fragments and ammonites extremely rare. - Trace fossils include Ophiomorpha and Diplocraterion. - Other sedimentary structures: o Lateral ripple-bundle sequences with rare and poorly developed reactivation surfaces. o Rare small-scale reversed ripple foresets with larger ripple bundles. o Mud drapes very rare. o Avalanche sand-tongue structures within tabular-planar foreset beds. o Bundled mud-sand tidal laminations rare in between channel bodies. o Cross-stratification unimodal to multimodal with bipolar components in the SW-NE and NW-SE directions. o Stacked sets of medium- and small-scale cross-stratification with intervening erosional surfaces reflecting composite dune development as in modern tidal systems. (Figures 2-1 – 2-8)
Properties of Tidal Channel Facies Overall - Exposures occur along the Missouri River gorge between Cochran Dam and Ryan Dam. - A complex of laterally and vertically stacked, clayey to clean quartzarenite; channel bodies up to 40 m thick and >5 km wide. - Erosionally overlies sand-wave facies; overlain by tidal bar to tidal flat and nonmarine Kk4 facies.
Channel Body Properties - Upward-fining sandstone and heterolithic fill. - Channel-body orientations approximately NE-SW. - Individual channel widths: several to > 20-m; channel thicknesses: meter to 10-m scale. - Bedding: subhorizontal to concave and ECS-like; over-steepened beds within slump blocks.
Sedimentary structures - Medium- to large-scale trough cross-stratification & small-scale ripple bedding. - Bank accretion bedding. - Mud drapes. - Wave ripples. - Vertically accreted spring-neap bundles of parallel lamination. - Slump blocks. - Trace fossils locally abundant; bioturbation fabric common, ranging up to 100%. - Paleoflow usually unimodal between SSW-SSE; rarely bimodal-bipolar with 2nd mode to north.
Subtidal Sand Bar/Flat Lithofacies (Figures 2-9 – 2-17)
Properties of Subtidal Bar/Flat Facies Overall - At Morony Dam: An 11-15 m-thick composite of up to five, vertically stacked, generally 2-4 m thick, upward-coarsening, mud (or heterolithic)-to-sand tabular units occurs directly above the basal Kootenai (Cutbank) fluvial sandstone. At Ryan Island: A relatively thin bar sequence (~5m) occurs above the sand wave facies. - The facies is directly overlain by tidal flat and coastal plain facies, or, locally truncated and overlain by estuary channel deposits. - Subhorizontal, low-angle, and convex-to-concave bedding indicates accretion upon wide, low relief, bar forms and adjacent subhorizontal surfaces. - Widespread, subhorizontal-to-slightly undulatory erosional surfaces truncate and occur within the Sunburst bar units. Sand-unit stacking and intervening erosion is consistent with episodic bar growth, abandonment (sand starvation), and tidal current breaching as occurs in modern estuary settings. - Other sedimentary structures o Localized low-relief, channel-shaped scours (m-scale width) with symmetrical fill. o Scattered wide (~5 m), low-amplitude (0.5-1.0 m) sets of trough cross-stratification. o Flaser bedding. o Rhythmic amagamated sets of small-scale ripple cross-stratification and parallel lamination. o Ripple cross-stratification usually unimodal to rarely bimodal-bipolar. o Bioturbation fabric common; trace fossils present but ichnospecies diversity low. Tidal Flat and Subtidal Facies (Figures 3-1 – 3-ll)
Properties of Tidal Flat / Subtidal Facies Overall - The facies occurs at the top of the Sunburst interval with a thickness range of ~1- 12 m. - The upper contact is transitional into oxidized mudstone and lithic sandstone of the nonmarine Kk4. - Sandstone- and mudstone-dominated units occur; each type of unit consists of upward-fining and upward-coarsening successions. - The composite vertical succession ranges from being a single upward-fining unit to multiple, upward-fining and coarsening units that culminate with fining into the Kk4. - Widespread, generally tabular, rhythmic beds and the presence of tide- and wave-associated structures indicate deposition in tidal and subtidal flat settings. - Vertical increases in mudstone, degree of bioturbation, organic debris, and oxidation reflect landward shallowing and energy decrease from intertidal mixed to supratidal mud-flat settings. - Thicker, coarser, amalgamated sandstone beds with a relative abundance of current ripple forms, medium-scale trough cross-stratification, and undulating or channel-like scour surfaces are consistent with modern subtidal sand-flat settings where tidal-current and wave-energy increases.
Other sedimentary structures - Flaser bedding and wavy bedding. - Bimodal cross-stratification. - Lingoid/lunate- and flat-crested wave-ripple bedforms. - Mud drapes. - Bundles of spring-neap parallel laminations. - Shallow erosional channels (gullies) and furrows (gutters) filled with sand and mud (produced on tidal flats during emergence and runoff). - Abundant bioturbation, commonly overprinting physical structures. Various ichnogenera including Planolites, Diplocraterion, and lingulid and horseshoe crab trace fossils (an estuarine tidal flat association).
(Figures 3-12 – 3-20) The following trace fossils, although mostly unidentified, serve to further establish a marine to brackish water setting.
Sand wave
Tidal Channel (Figure 3-15)
Tidal Flat
Tidal Channel Subtidal Bar/Flat
Berkhouse, G.A., 1985. Sedimentology and diagenesis of the Lower Cretaceous Kootenai Formation in the Sun River Canyon area, northwest Montana: M.S. thesis, Indiana University, Bloomington, IN, p. 1-26. Burden, E.T., 1984. Terrestrial palynomorph biostratigraphy of the lower part of the Mannville Group (Lower Cretaceous), Alberta and Montana, in Stott, D.F., and Glass, D.J., eds., The Mesozoic of Middle North America, Canadian Society of Petroleum Geologists Memoir 9, p. 249-270. Farshori, M.Z., and Hopkins, J.C., 1989. Sedimentology and petroleum geology of fluvial and shoreline deposits of the Lower Cretaceous Sunburst Sandstone Member, Manville Group, southern Alberta: Bulletin of Canadian Petroleum Geology, v. 37, p. 371-388. Hopkins, J.C., 1985. Channel-fill deposits formed by aggradation in deeply scoured, superimposed distributaries of the Lower Kootenai Formation (Cretaceous): Journal of Sedimentary Petrology, v. 55, p. 42-52. Mudge, M.R., and Rice, D.D., 1982, Lower Cretaceous Mount Pablo Formation, northwest Montana: U.S. Geological Survey Bulletin, v. 1502 D, p. 1-19. Vuke, S.M., 1987, Marine tongue in the middle Kootenai Formation north of Helena, Montana, in Berg, R.B., and Breuninger, R.H., eds., Guidebook of the Helena Area, west-central Montana, Special Publication 95: Tobacco Root Geological Society. Twelfth Annual Field Conference, p. 63-64. Walker, T.F., 1974, Stratigraphy and depositional environments of the Morrison and Kootenai Formations in the Great Falls area, central Montana: PhD thesis, University of Montana, p.195.
|
