Mudstone facies types Mudstone facies

into glauconitic sediments erosive, graded beds indicate a storm-influenced deposition.

This storm-influenced origin of bioclastic carbonates in the Redwater Shale Member (Sundance Formation) was also concluded by SPECHT & BRENNER (1979).

Stratigraphic distribution: The biowackestone microfacies is represented in the Twin Creek Limestone, the Carmel Formation and Stump Formation in western Wyoming, eastern Idaho and northeastern Utah. Furthermore, in the Sundance Formation in central Wyoming and in the Rierdon Formation in south-central Montana.

3.1.2.4 Mudstone facies types

Figure 3-6: Oval mudstone concretions are a widespread feature within the Redwater Shale Member of the Sundance Formation and the Stump Formation in Wyoming and adjacent areas. Length of hammer 32 cm.

Biomudstone facies

Samples: EM 6, HC 1, HU 2, HR 10b, MIN 11, SR 1, SR 2, SPC 6, TC 6, THI 10, W 1, W 2.

Components: The biomudstone microfacies contains more than 10 % particles and is composed of varying amounts of biogen particles and trace amounts of non-biogen particles. The major biogen particles make up approximately 10 Vol.% of the biomudstones and comprise 1-2 mm long pelecypod fragments with thick micritic envelopes and crinoid fragments, 0,8 mm in diameter. Further, fragments of thin-shelled gastropods, unidentified bivalves, foraminifers, articulated and disarticulated ostracods, and scaphopods are present. The non-biogen particle spectrum is composed of superficial ooids, 0,5 mm in diameter, peloids <0,25 mm in diameter and rounded intraclasts of dark brown micrite, <0,25 mm in diameter. The detritus content comprises quartzose and/or allochthonous glauconitic grains in silt to fine sand-size. The degree varies between 1 and 7,5 Vol. %. The glauconite is either fresh medium-green or greenish-brown due to alteration.

Matrix: Grayish to dark-brown micrite or pseudosparite. Syntaxial cements are developed around echinoderm particles.

Texture: Matrix-supported.

Bedding and sedimentary structures: The biomudstone microfacies displays in outcrop thin-bedded, wavy lamination as shown in Figure 3-7. It forms massive, up to 50 m thick stratal packages, interbedded with mudstone facies types as at sections Thistle (THI) and Twin Creek (TC). Further, the microfacies is exposed as bored, oval concretions (see Figure 3-8), 3-40 cm in diameter, embedded in glauconitic shales of the Redwater Member of the Sundance Formation as at sections Minnekatha (MIN), Hulett (HU), Elk Mountain (EM), Alcova Reservoir (AR), within the Sawtooth Formation in northeastern Montana as found at section Swift Reservoir (SR) and within the Carmel Formation as at section Whiterocks Canyon (W).

Interpretation: The biomudstones are genetically and spatially related to the mudstone microfacies. A subtidal origin as calcareous mud is also suggested for the thick, distal mudstone packages in the Twin Creek Limestone in correspondence with the interpretation of IMLAY (1967). The occurrence of the biomudstone microfacies as oval concretion indicates a deposition under low-energetic conditions below storm wave base.

Stratigraphic distribution: The biomudstone microfacies is represented in the Sundance Formation in Wyoming and South Dakota. Further in the Twin Creek Limestone along the Wyoming-Idaho border, the Sawtooth Formation in northwestern Montana and the Carmel Formation in northeastern Utah.

Figure 3-7: Thin-bedded biomudstones in the Sliderock Member of the Twin Creek Limestone at section South Piney Creek (SPC). The lense cap is 6 cm in diameter.

Figure 3-8: Oval biomudstone concretion at the base of a storm-deposited sandstone bed in the Redwater Shale Member of the Sundance Formation at section Hampton Ranch (HR), Bighorn Basin/WY. Head of Jacob stick is approximately 15 cm long.

Detritusmudstone facies

Samples: BE 4, BE 5, FG 4, FG 10, FG 18, FG 19, FG 20, FG 21, FG 24, FH 5, HE 1, HR 1, HR 10a, HU 4, HU 12, LB 2, LW 2, LW 8, MIN 8, PF 2, SR 5, SBC 1, THI 31, THI 24, THI 23, THI 25, THI 26, THI 27, THI 20, THI 21, THI 11, THI 12, THI 13, THI 14, TR 10, V 7, W 4, W 8.

Components: The detritusmudstone microfacies contains trace amounts of biogen particles and trace amounts of non-biogen particles. The detritus content comprises quartzose and/or allochthonous glauconitic grains (THI 24, FH 5, HU 12, FG 24) in silt to fine sand-size. The degree of quartzose grains varies between 1 and 10 Vol. % and of glauconitic grains between 1 and 4,5 Vol. %. The glauconite is either fresh medium-green or greenish-brown due to alteration. The major biogen particles make up approximately 5 Vol.% of the detritusmudstones and comprise 1-2 mm long pelecypod fragments with thick micritic envelopes and crinoid fragments, 0,7-1 mm in diameter. Further, fragments of thin-shelled gastropods and microschill are present. The non-biogen particle spectrum is composed of peloids, <0,25 mm in diameter.

Matrix: Grayish to dark-brown micrite or pseudosparite.

Texture: Matrix-supported.

Bedding and sedimentary structures: The detritusmudstones are commonly structureless. In some samples poorly preserved, tube-like structures, up to 4 mm in diameter are present and probably document bioturbation. The detritusmudstone microfacies forms massive stratal packages up to 35 m thick interbedded with various mudstone facies types at sections Big Elk Mountain (BE), Little Water Creek (LW), Thistle (THI). Alternatively, the microfacies occurs as 0,2-0,4 m thick layers intercalated into glauconitic shales as at sections Hampton Ranch (HR), Hulett (HU) or Stockade Beaver Creek (SBC). The microfacies corresponds to the “terrigenous mudstone facies”

as described from the southern “Sundance Basin” by BLAKEY et al. (1983) in respect to particle spectrum and absence of bedding structures.

The microfacies is exposed as bored, oval diastemic cobbles, 3-40 cm in diameter, embedded in glauconitic shales of the Sundance Formation as at sections Thompson Ranch (TR), Minnekatha (MIN), Hulett (HU), Hampton Ranch (HR), Stockade Beaver Creek (SBC), within the Sawtooth Formation in southwestern Montana as at section Little Water Creek (LW), in northeastern Montana as at section Swift Reservoir (SR), within the Stump Formation as near Vernal (V) and in the Twin Creek Limestone.

Interpretation: The “terrigenous mudstone facies” was deposited in a variety of low-energetic settings in restricted to normal marine environments as proposed by BLAKEY et al. (1983). In the southern “Sundance Basin”, the origin of this microfacies is non-specific and can only be determined in context with adjacent facies types (BLAKEY et al. 1983).

This is also true for other areas of the basin. The detritusmudstones in the study area are genetically and spatially related to the mudstone and the biomudstone microfacies of subtidal origin.

Stratigraphic distribution: The detritusmudstone microfacies is represented in the Sundance Formation in Wyoming and South Dakota. Further in the Twin Creek Limestone, in the Sawtooth Formation in southwestern Montana, the Rierdon Formation in northwestern Montana, and the Carmel Formation in northeastern Utah.

Laminated mudstone facies

Samples: LW 9, RC 6, TC 1, TC 2, TC 3, THI 28, THI 32, T-T 2.

Particles: The major grain constituents of the laminated mudstone microfacies are trace amounts of microschill and silt to fine sand-sized arenaceous detritus.

Matrix: Inhomogenous light-brown to grayish-brown micrite and pseudosparite.

Bedding and sedimentary structures: The laminated mudstones display mm-thick, flaser to convolute layers in wavy lamination. Further, cracked mudflakes and salt crystal casts are present. In outcrop the microfacies is associated with red shales and siltstones as at sections Twin Creek (TC) and Thistle (THI). In respect to sediment structures the laminated mudstone microfacies is equivalent to the “algal-laminated dolomicrite facies”

as described from the southern “Sundance Basin” by BLAKEY et al. (1983) and an algal limestone facies described by JOHNSON (1992) from southern Wyoming.

Interpretation: The sediment structures and red bed facies types associated with the laminated mudstones suggest deposition in low-energetic environment in the vicinity of a carbonate-dominated shoreline.

Stratigraphic distribution: The laminated mudstone microfacies is represented in the Sundance Formation in northeastern Wyoming and South Dakota. Further in the Twin Creek Limestone along the Wyoming-Idaho border and the Rierdon Formation in southwestern Montana.

A: Oograinstone microfacies. Sample DH 1 from the Twin Creek Limestone at section Devils Hole Creek (scale is 2 cm for the lower side of the photo, bright field).

B: Oograinstone microfacies. Sample TF 3 from the Twin Creek Limestone at section Thomas Fork Canyon. (scale is 1,2 cm for the lower side of the photo, bright field).

C: Oobiograinstone microfacies. Sample HR 5 from the Sundance Formation at section Hampton Ranch (scale is 6,3 mm for the long side of the photo, bright field). Planar stratification of ooid-rich and sand-rich layers. The ooid-rich layers contain pelecypod and crinoid fragments.

D: Oobiograinstone microfacies. Sample SPC 4 from the Twin Creek Limestone at section South Piney Creek (scale is 6 cm for the long side of the photo, bright field). Ooids are mixed with crinoid fragments.

E: Oobiograinstone microfacies. Sample V 10 from the Stump Formation at section Vernal (scale is 6,3 cm for the long side of the photo, bright field).

Ooids are mixed with pelecypod fragments.

F: Oobiograinstone microfacies. Sample FG 28 from the Stump Formation at section Flaming Gorge (scale is 2,1 cm for the long side of the photo, bright field). Ooids are mixed with poorly sorted pelecypod fragments, intraclasts and gastropod fragments. The pelecypods are rounded and display thick micritic envelopes.

Plate 1: Photographs from thin-sections of oolitic grainstone microfacies types.

A: Biograinstone microfacies. Sample AR 5 from the Sundance Formation at section Alcova Reservoir (scale is 1,1 cm for the long side of the photo). The particles (pelecypods and crinoids) are poorly sorted and densely packed. The sample has a strong rudstone-like texture.

B: Biograinstone microfacies. Sample LW 1 from the Sawtooth Formation at section Little Water Creek (scale is 0,8 cm for the long side of the photo). The sample consists mainly of poorly sorted, subrounded pelecypod fragments and disarticulated crinoidal columns.

C: Biopackstone microfacies. Sample LB 5 from the Twin Creek Limestone at section La Barge Creek (scale is 1,1 cm for the long side of the photo). The sample shows a strong affinity to a rudstone texture due to the poor sorting of the pelecypod fragments.

D: Biopackstone microfacies. Sample MIN 10 from the Sundance Formation at section Minnekatha (scale is 1,1 cm for the long side of the photo). The sample shows a strong affinity to a rudstone texture. The particle spectrum comprises crinoidal columns and pelecypod shells with micritic envelopes.

E: Biowackestone microfacies. Sample W 3 from the Carmel Formation at section Whiterocks Canyon (scale is 1,1 cm for the long side of the photo). The particles are poorly sorted and not oriented.

F: Biowackestone microfacies. Sample AR 9 from the Sundance Formation at section Alcova Reservoir (scale is 1,1 cm for the long side of the photo).

Densely packed pelecypod fragments grade upward into convolute laminated fine-grained sandstones.

Plate 2: Photographs from thin-sections of grainstone, packstone and wackestone microfacies types.

A: Biowackestone microfacies. Sample HU 6 from the Sundance Formation at section Hulett (scale is 1,2 cm for the long side of the photo). The particles are poorly sorted and not oriented. Note the large, well rounded oyster fragment in lower half of photo.

In some samples up to 25 mm long particles make up more than < 10 Vol. % and superimpose a floatstone-like texture.

B: Mudstone microfacies. Sample FH 6 from the Sundance Formation at section Freezeout Hills (scale is 1,2 cm for the long side of the photo). This sample stems from a mudstone concretion in the Redwater Shale Member.

Plate 3: Photographs from thin-sections of wackestone and mudstone microfacies types.