water below fairweather wave base (FWWB). The low-energetic sedimentation is frequently effected by storm events and bad weather periods. The ichnofacies assemblage becomes Cruziana-dominated. The sediment facies is mud-dominated and represented by the shale lithofacies (shale lf) and various mudstone microfacies types (biomudstone, detritusmudstone, mudstone). Further, storm-deposited biowackestones and sandy beds are interbedded. In context with the definition of BURCHETTE &
WRIGHT (1992) the mid-ramp is reflected by the biowackestone microfacies types, while mudstone microfacies types typify the outer ramp.
4.5 Ramp models for differing basin configurations in the “Sundance Basin”
Ramp configurations occur in a variety of sedimentary basins, but are “best developed where subsidence is flexural and gradients are slight over large areas, as in foreland, cratonic-interior and along passive margins” (BURCHETTE & WRIGHT 1992: 3). This implies that besides the hydrodynamic regime the tectonic configuration and the influence of subsidence are of special importance for the development of ramp systems (HANFORD
& LOUCKS 1993). As pointed out by READ (1982; 1985), a common phenomenon in the
geologic history is the evolution of a homoclinal ramp toward a distally steepened configuration. In consequence, it would be necessary to progressively modify ramp models in relation to particular evolutionary tectonic stages of a sedimentary basin. The distal steepening of ramp systems might be either tectonically driven (differential subsidence), inherited or occur due to intrinsic processes (differential sedimentation). The ramp classification of READ (1982; 1985) offers the opportunity to modify homoclinal ramp models during transformation toward distally steepened models.
This relation will be of special significance for the present study. As will be demonstrated, two major geometric settings can be distinguished during evolution of the “Sundance Basin” that require two ramp models:
• A homoclinal ramp model for symmetric basin configurations, characterized by lithologic mixed deposystems, low morphological gradients, limited accomodation space, and a specific energy zonation that is typified by a shoreline-detached high-energy facies.
• A distally steepened ramp model characterized by an asymmetric geometry. This model is composed of a proximal, siliciclastic-dominated domain that grades laterally into distal, carbonate-dominated domains. The morphological gradient steepens distally toward the developing basin slope.
In both ramp models the energy zonation is caused by gradually decreasing hydrodynamic energy toward the offshore/outer ramp zone. The most significant contrasts between the two ramp configurations are confined to the spatial distribution of siliciclastic and carbonate sediments.
4.5.1 Homoclinal ramp model
The homoclinal ramp model describes a prominent configuration during the evolution of the “Sundance Basin”. Homoclinal ramp settings were dominant during deposition of the First (C I) and Fourth Marine Cycle (C IV). In contrast to a distally steepened ramp configuration the offshore/outer ramp zone II deposits are thin and storm interbeds occur with a much higher frequency in the stratal record. Further, siliciclastic and carbonate sediments are spatially associated and occur in all depozones of the ramp. The carbonate-siliciclastic homoclinal ramp model is illustrated in Figure 4-10 A.
4.5.2 Distally steepened ramp model
The distally steepened ramp model is corresponding to the homoclinal ramp model in respect to the principal facies and energy zonation. In contrast to the homoclinal ramp model the distal deposits in the offshore/outer ramp zone II are much thicker. Moreover, the distal portion of the ramp is differentiated and mid-ramp sediments (biowackestones) can be distinguished from outer ramp mudstones. The fairweather wave base (FWWB) is
delineated by the massive build up of oolite facies types. A very strong contrast to the homoclinal ramp model is expressed in the pronounced spatial separation of siliciclastics and carbonates. The distally steepened ramp model is characterized by siliciclastic sedimentation in the proximal part with low depositional gradients and carbonate sedimentation in the distal part on the mid- and outer ramp. The carbonate-siliciclastic distally steepened ramp model is illustrated in Figure 4-10 B. This configuration is favored by the asymmetric spatial subsidence behavior within the “Sundance Basin” and expressed in the Second (C II) and Third Marine Cycle (C III). More precisely, a distally steepened ramp configuration can be proposed for the developing stage of the “Utah-Idaho trough”.
4.6 Basinwide facies context
Facies models for siliciclastic and carbonate depositional settings as shown in Figure 4-11 are developed for the Carmel Formation by BLAKEY et al. (1983). The existence of a southward adjacent facies model provides the opportunity to control the proposed facies mosaic of homoclinal to distally steepened ramp models and place them in a basinwide context. According to BLAKEY et al. (1983), the narrow, confined nature of the “Carmel seaway”, that occupied the “Utah-Idaho trough” and the gentle slope of the adjacent coastal plain resulted in extremely wide facies belts. As noticed by BLAKEY et al. (1983), TUCKER & WRIGHT (1990) and EINSELE (1992), no modern analogues for such configurations are known. Basinwide, the facies models introduced by BLAKEY et al.
(1983), for the southern “Sundance Basin” and the facies zonation proposed for the central and northern portions in this study are corresponding in respect to their morphological gradients, hydrodynamic conditions, facies zonation (lithofacies and ichnofacies), resemblance of analyzed carbonate microfacies types (see chapter: 3.1.1, Carbonate microfacies analysis) and distally increasing water depths.
According to BURCHETTE & WRIGHT (1992) and SARG (1988), the basinward slope zone in distally steepened ramps may display a slope apron and slump structures. Such structures that indicate rapid mass transport were neither observed during field work nor reported by previous workers. There are two possible explanations: Either the basin slope was gentle, as shown in the carbonate facies model A of BLAKEY et al. (1983), so that mass transport was not induced or potential slump deposits were subsequently reworked by storms.
Arenicolites-Ichnofacies
Skolithos-Ichnofacies
Cruziana-Ichnofacies
Color code for
depositional environments
continuousdiscontinuous storm deposits subtidal
high-energy facies belt shallow marine/peritidal peritidal
lagoonal sabkha terrigenous
Abbreviations for lithofacies types (lf = lithofacies)
LX-lf: large-scale cross-bedded lf LL-lf: low-angle laminated lf Gl-lf: glauconitic lf, Oo-lf: oolite lf, WR-lf: wave-rippled lf,
L-Fb-lf: lenticular to flaser bedded lf silt lf: silty lf,
shale lf: shale lf
B
(not to scale, no paleogeographic implications) (not to scale, no paleogeographic implications)A
Figure 4-10: A: Homoclinal ramp model for the mixed carbonate-siliciclastic depositional system within the
“Sundance Basin” and arrangement of depositional zones 0, I and II; B: Distally steepened ramp model for the mixed carbonate-siliciclastic depositional system within the “Sundance Basin” and arrangement of depositional zones 0, I and II.
Figure 4-11: General facies model, proposed by BLAKEY et al. (1983) to display the depositional settings during the Middle Jurassic in the southern “Sundance Basin”. A: for the carbonate-dominated facies, B: for the terrigenous-dominated facies (from BLAKEY et al. 1983).
4.7 Facies analysis and modelling characteristics
Based on the facies analysis, 11 carbonate microfacies, 10 siliciclastic lithofacies and an evaporitic facies can be distinguished in the “Sundance Basin” fill. The depositional environments of these facies types are characterized by high-energetic to low-energetic hydrodynamic conditions. The sedimentary facies interpretation is supported by the observed ichnofacies that describe hydrodynamic high-energetic versus low-energetic environmental conditions. The facies zonation describes spatially adjacent depozones with a specific offshore protracted decrease of energy gradients. This offshore-directed decrease of energy gradients is associated with increasing water depths. The continuous hydrodynamic zonation is temporarily interrupted by storm events. Morphological gradients within the “Sundance Basin” primarily controlled this continuous hydrodynamic zonation. Facies models for the “Sundance Basin” are best described by homoclinal and distally steepened ramp settings. Well expressed interfaces in the sedimentary facies spectrum on these ramps are the fairweather wave base (FWWB) and the storm wave base (SWB) that mark boundaries between differing depozones and are expressed by diagnostic sediment structures like large-scale bedding, hummocky
cross-lamination, herring-bone cross-bedding, various wave ripple laminations, planar bedding, and coquinoid beds in the investigated stratigraphic column. Due to the moderate morphological gradient the resulting depozones 0, I and II are broad and extremely wide.
In depozone 0 terrigenous and sabkha sedimentation is dominant. Zone I includes shoreface-foreshore environments above fairweather wave base (FWWB), while zone II is typified by offshore-mid- to outer ramp settings above storm wave base (SWB).
Temporary modifications in the geometric basin configuration of the “Sundance Basin” by tectonic activity on the western edge of the North American craton suggests temporarily alternating ramp models (homoclinal and distally steepened) to comprehensively describe the spatial arrangement of carbonate-siliciclastic depositional environments.
5 Facies and allostratigraphic correlation in the “Sundance Basin”
To display the distribution and correspondence of facies types and bounding surfaces within the “Sundance Basin” seven transections through the outcrop area of Jurassic formations were constructed. These 2-dimensional projections provide the basis for the compilation of facies maps and 3-dimensional fence diagrams of the entire study area.
Figure 1-1 shows the position of the seven transections. Identified facies types were grouped as facies associations in respect to the ramp environments of depozone 0, I and II (see chapter: 4, Facies models). The facies types and associations were assigned with a color code and correlated between measured outcrop sections under consideration of (a) the allostratigraphic framework provided by the identified Jurassic unconformities of PIPIRINGOS & O’ SULLIVAN (1978) and additional subordinate interfaces and (b) the biostratigraphic framework defined by IMLAY (1980) within the major sedimentary cycles.
The color code and facies associations are listed in the explanation chart in Figure 5-1.
If necessary the outcrop grid was extended with additional data and supplementary facies types from previous publications to maintain control on thickness trends, facies correspondence, spatial extent, and stratigraphic position of bounding unconformities (see chapter: 3.6, Supplementary facies types). For this purpose, outcrop sections described by IMLAY (1967; 1980), MORITZ (1951), PIPIRINGOS (1957), AHLBRANDT & FOX (1997), BÜSCHER (2000), SCHMUDE (2000), FILIPPICH (2001), SPRIESTERSBACH (2002), and DASSEL (2002), were compared and placed in context with the examined outcrop sections.
5.1 2-dimensional facies correlation
North-south oriented transections A – A’ to C – C’
The transections A – A‘ to C – C‘ are north-south oriented. Due to their individual 2-dimensional facies distribution the three transections are discussed separately.
Transection A – A’
The transection A – A’ in Figure 5-2 extends from the northernmost outcrop at section Swift Reservoir (SR) in Montana through the Sawtooth Range of southwestern Montana (section LW) into the “Overthrust Belt” and runs along the Wyoming-Idaho border (section BE – TC) southward to the southern flank of the Uinta Mountains in northeastern Utah.