Berthierine/chamosite, corrensite, and discrete chlorite from evolved verdine and evaporite-associated facies in the Jurassic Sundance Formation, Wyoming

Late Jurassic sandstones of the shallow-marine Sundance Formation contain authigenic chlorite minerals that occur as rosette-like pore fillings of interstratified berthierine/chamosite (B-C) and honeycomb-like pore linings of corrensite and discrete chlorite. B-C is nearly ubiquitous in Sundance sandstones, but is absent near the top of the formation, whereas corrensite and discrete chlorite were detected only in uppermost Sundance sandstones, within 4 m of the contact with the overlying nonmarine Morrison Formation. Glauconite grains are common and occur as laminae along bedding planes and cross-beds, indicating reworking and deposition as clasts in tidally influenced regimes. The mineralogical, chemical, and morphological properties of the B-C and corrensite indicate that they are authigenic and formed during burial diagenesis from precursor minerals, odinite in the case of B-C, and saponite in the case of corrensite and discrete chlorite. Odinite has been recognized in numerous shallow-marine sands of the Holocene verdine facies, and the shallowmarine conditions associated with Sundance deposition would have been ideal for odinite formation. Saponite commonly forms in aeolian and evaporitic environments, implying that the saponite precursor to corrensite and discrete chlorite formed in uppermost Sundance sands that were exposed to an influx of oxidizing groundwater following regression of the Sundance sea. X-ray diffraction (XRD) indicates that the proportion of 7 Å B layers in B-C ranges from 5 to 28%, and chemical analysis by scanning electron microscope-energy dispersive X-ray spectrometry (SEM-EDS) indicates positive correlation between %B and Fe/(Fe + Mg). The polytype of the B-C is Ibb, and the corrensite and discrete chlorite are disordered IIb. These are the expected polytypes and %B for sandstones exposed to burial diagenetic conditions of 3000–4000 m and temperatures 90–120 ∞C. RYAN AND HILLIER: FACIES RELATIONSHIPS OF Fe-RICH CLAYS 1608 proximity of the depositional environment to rivers (Porrenga 1967; Odin 1985; Odin and Sen Gupta 1988; Odin 1990). In contrast to the shallow depth of formation of odinite, glauconite commonly forms at the sediment-seawater interface at the shelf-slope break, where sedimentation rates are low, water depths are >100 m, and temperatures are 10–15 ∞C (Odin 1988). Glauconite is also less restricted geographically, forming at latitudes as high as 60∞. A good example of the relationship between the verdine and glaucony facies is given by Porrenga (1967), who examined green granules in Holocene shallow marine sediments from the Niger and Orinoco deltas. Odinite (originally termed chamosite by Porrenga) and glauconitic smectite were identified in sediments sampled from water depths of 10–50 and 125–250 m, respectively. The odinite was described as Fe-rich and poorly ordered with a dominant 7 Å reflection; the glauconitic smectite contained 70% expandable layers and its XRD characteristics resembled Fe-rich smectite. Subsequent research by Odin and coworkers (e.g., Odin and Giresse 1972; Odin 1985, 1988, 1990; Odin et al. 1988) documented similar relationships between odinite and glauconitic minerals in the Congo River delta, on the continental shelf between the Orinoco and Amazons Rivers, in New Caledonia, and in other shallow-marine environments close to the mouths of rivers (Odin 1990). More recent studies have documented similar relationships in marine sediments from off-shore India and Australia (e.g., Rao et al. 1995; Kronen and Glenn 2000). In this paper, we seek to assess relationships between interstratified berthierine-chamosite (B-C) rosettes, which are nearly ubiquitous in Sundance Formation sandstones, and porelining corrensite and discrete chlorite, which are present only in sandstones from the upper 4 m of the Sundance Formation. We also address the presence of glauconite, which is also nearly ubiquitous in the Upper Sundance Formation. XRD, SEM-EDS, and stratigraphic analysis are used to assess the possibility that the Sundance Formation represents an evolved analog of the verdine facies, and to further assess the idea that the corrensite originated as saponite that formed in the presence of oxidizing, evaporitic groundwater produced by marine regression. In addition to analyzing the origins of these clays, we also examine the chemical composition, mixed-layering, and polytypism of the B-C, corrensite and discrete chlorite, parameters that are often used to assess diagenetic and metamorphic grade (e.g., Cathelineau 1985; de Caritat et al. 1993; Jiang et al. 1994; Hillier 1994; Ryan and Reynolds 1996; Shata and Hesse 1998). MATERIALS AND METHODS Sandstone beds of the upper Sundance Formation (Late Jurassic) were sampled from outcrops in the Bighorn Basin of north-central Wyoming and the eastern Wind River basin. The Bighorn Basin is located between 43.5–44.5 ∞N and 107–108.5 ∞W, and the Wind River Basin is located between 42.5–43.5 ∞N and 107–108.5 ∞W. The Sundance Formation is divided informally into lower and upper members, with each representing a major transgressional/regressional marine sequence (Picard 1993). The focus of this study is the tabular sandstone beds of the Upper Sundance Formation, which were deposited in tidal flats, tidal inlets, and sandy shoals (Uhlir et al. 1988). Sundance sediments were deposited at a paleolatitude of ~35 ∞N in a warm and arid to semi-arid climate (Picard 1993). The Upper Sundance Formation in the study area varies from 15 to 30 m thick (Uhlir 1987; Uhlir et al. 1988). In one case (Lander Dome, eastern Wind River Basin), a well-exposed stratigraphic section was sampled from the lower part of the Upper Sundance Formation up through the contact with the overlying fluvial and lacustrine Morrison Formation. Stratigraphic relationships of these formations are shown on Figure 1. Estimated burial depth of the Sundance Formation prior to the Laramide Orogeny is 3000–4000 m and is based on: (1) current 3000 m burial depth of Sundance Formation obtained from numerous oil wells in the central Bighorn Basin and elsewhere (Peterson 1957), and (2) stratigraphic thicknesses of pre-Laramide sedimentary rocks atop the Sundance Formation in the study area (Steidtmann 1993). Under a normal geothermal gradient of 25 ∞C/km and a surface temperature of 20 ∞C, this implies maximum diagenetic temperatures of approximately 90–120 ∞C. Specimens were crushed gently and without grinding, and then sonified in a water bath to enhance disaggregation. The 1–2 mm and <1 mm fractions were obtained by gravity settling in Atterberg cylinders. In order to assess mixed-layering of glauconite and chlorite minerals, oriented mounts of these two size Floodplain, lagoonal, aeolian deposits