Cleavage Development in Dolomite of the Elbrook Formation, Southwest Virginia
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Cleavage development in dolomite of the Elbrook Formation, southwest Virginia JANET SCIIV^l^ I'l'^F.R \ CAROL SIMPSON 1 department of Geological Sciences, Virginia Polytechnic Institute & State University, Blacksburg, Virginia 24061 ABSTRACT collapse of phyllosilicates and quartz to form clay seams (Beach, 1979, 1981; Gray, 1981; Marshak and Engelder, 1985). Argillaceous material Well-develo])ed cleavage in argillaceous Cambrian Elbrook For- has also been found to exert some control on cleavage formation, and a mation dolomite lias been analyzed petrographically and chemically to positive correlation between argillite content and cleavage intensity has evaluate the defoimation mechanisms responsible for cleavage forma- been observed such that well-cleaved limestones generally contain more tion and the lithologic controls on these mechanisms. Two main types than 10% clay (Nickelsen, 1972; Groshong, 1975; Gray, 1979; Marshak, of cleavage occur: moderately spaced cleavage defined by coarse, 1983; Tapp, 1983; Mitra and others, 1984). The effect of clays on the continuous, wavy domains, and narrowly to closely spaced cleavage behavior of dolomite has not been previously documented. defined by fine, discontinuous, anastomosing domains. This paper describes the microstructures and chemical changes asso- The two moist important deformation mechanisms during cleav- ciated with cleavage development in dolomite of the Cambrian Elbrook age formation were pressure solution and mechanical rotation. Evi- Formation near the base of the Pulaski thrust sheet. A model for cleavage dence for pressure solution includes off-set sedimentary structures, in impure dolomite is presented and is compared with existing models for grain dissolution, syn-cleavage fibrous veins, and accumulations of limestones. Observations are based on examination of polished thin sec- quartz, phyllosilicates, and pyrite in cleavage domains. Pressure solu- tions, using standard transmitted light microscopy and an ARL-SEMQ tion was enhanced by the presence of clay minerals. electron microprobe. All samples were cut perpendicular to bedding and A model for cleavage development in dolomite is presented in cleavage. which cleavage initiates by the dissolution of dolomite grains at dolomite-phyllosilicate grain boundaries. Rotation of phyUosilicate GEOLOGIC SETTING grains concomitant with dolomite removal resulted in a strong pre- ferred orientation of layer silicates within cleavage domains. Finer The Pulaski thrust fault, one of four major southeast-dipping thrust cleavage domains coalesced to form coarser domains as the interven- faults in the southern Appalachian Valley and Ridge province, places ing lithon dolomite was dissolved. This model is comparable to those deformed Cambro-Ordovician sediments over deformed Cambrian to previously proposed for cleavage development in argillaceous lime- Mississippian sedimentary rocks of the Saltville thrust sheet (Fig. 1). Tim- stones and siltstones. ing of the initial emplacement of the Pulaski thrust sheet is poorly con- strained as the youngest rocks in the thrust sheet are Middle Ordovician INTRODUCTION (Schultz, 1983). Final emplacement occurred during the late Paleozoic (Alleganian) (Schultz, 1983). Near the study area, the Pulaski thrust sheet Well-develop* ;d spaced cleavage is commonly observed in lime- is 1,500 m thick (Schultz, 1983). Conodont alteration (Epstein and others, stones, but it is less commonly reported in dolomite. Experimental and 1977) and illite crystallinity (Schweitzer, 1984; Schultz, in press) indicate field data for deformed pure calcite and pure dolomite show that the two that the maximum temperature at the base of the thrust sheet was between minerals deform differently. Experimental studies indicate that calcite be- 200 and 350 °C. comes weaker and more ductile with increasing temperatures, whereas The Elbrook Formation occupies a structural position near the base dolomite increases in strength and deforms by brittle fracturing up to -400 of the Pulaski thrust sheet. In the lower part of the formation, thinly °C (Higgs and Handin, 1959; Turner and Weiss, 1963; Wenk and Shore, laminated dolomite and calcareous mudstones alternate with more mas- 1975). In a study of naturally deformed carbonates, it was found that for sive dolomites (Schultz, 1983). The dolomite is highly fractured and the same grain size, dolomite is stronger than limestone at shallow crustal veined in places (Cooper, 1939; Cooper and Haff, 1940), and lccally levels (Hugman and Friedman, 1979). Thus at low temperatures, a pure displays well-developed disjunctive, spaced cleavage, especially in the dolomite usually shows brittle behavior, whereas a limestone generally lowest, highly argillaceous horizons (Schultz, 1983). Cleavage is found in deforms by twinning and pressure solution to produce a spaced cleavage. Ramsay (1967) class IB, 1C, or 2 upright and inclined folds that trend Argillaceous dolomites in the central Valley and Ridge province, however, southwest-northeast (Fig. 2). It is axial planar to slightly convergent or are known to contain spaced cleavage (Helmstaedt and Greggs, 1980; defines southeast-dipping cleavage fans with planar to curved axial sur- Tapp, 1983), as aire similar lithologies in the Pulaski thrust sheet of faces (Fig. 2). Cleavage density is generally greatest in hinge regions, southwest Virginia. slightly less on overturned limbs, and lowest on upright limbs. The trace of Spaced cleavage and tectonic stylolites in impure limestones have been the cleavage is often continuous, with planar to anastomosing morphology, attributed to removal of calcite by solution processes and consequent and it is commonly refracted across lithologic contacts. Geological Society of America Bulletin, v. 97, p. 778-786,6 figs., June 1986. 778 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/6/778/3445197/i0016-7606-97-6-778.pdf by guest on 27 September 2021 a NW meters Pulaski Fault + 600 0 2 km j (_ Figure 1. (a) Location of study area in southern Appalachian Valley and Ridge province. Pulaski thrust sheet marked by stipples. Boxed area enlarged in b. (b) Geologic map of study area (modified from Schultz, 1983). Sample collection localities marked by stars. P.M.W. = Price Mountain Window; (c) Cross section X-Y (from Schultz, 1983). Cr = Rome Formation, €b = Broken Formation of Schultz (1983), Ce = Elbrook Formation, Cc = Conococheague Formation. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/6/778/3445197/i0016-7606-97-6-778.pdf by guest on 27 September 2021 780 SCHWEITZER AND SIMPSON Q NW SE 3 meters i meter Figure 2. Outcrop sketch and orientation data of deformed Elbrook Formation near Pulaski, Virginia, (a) Structure section showing folds; in northwest portion of outcrop, f = faults, medium lines = bedding, fine lines = cleavage, (b) Enlargement of boxed area in 2a showing overturned syncline with convergent spaced cleavage. Sample localities numbered, (c) Equal-area projection of orientation data from fold illustrated in 2b. Poles to bedding ( • ), poles to cleavage (A). Best-fit 7r-circle (dashed) and /?-axis (®) determined by inspection. CLEAVAGE TERMINOLOGY narrow, close, moderate, or wide (Fig. 3a). Cleavage morphology is de- scribed in terms of small-scale (Fig. 3b) and large-scale (Fig. 3d) Spaced cleavage, the most common type in carbonates, can be further topography of cleavage domains as well as domain length (Fig. 3c] and classified as disjunctive or crenulation cleavage (Powell, 1979). In the width (Fig. 3e). Elbrook Formation, the cleavage is predominantly spaced disjunctive, with rare crenulation cleavage. Disjunctive cleavage consists of cleavage UNDEFORMED ELBROOK FORMATION domains and micro lithons. The domain is the plane or zone which has been altered by deformation and is generally defined by concentrations of The lithology of the lithons in cleaved Elbrook Formation is the same insoluble material, elongated mineral grains, and foliated phyllosilicates as that of the undeformed rock away from the cleaved regions. The unde- (Powell, 1979). The microlithon is the material between the cleavage formed lower Elbrook Formation is a thinly laminated (0.75-5.0 mm), domains. Cleavage s pacing in the Elbrook Formation is commonly several fine-grained (9-60 ¡xm) ferroan dolomite with interbedded calcareous millimetres, and thus the term "microlithon" is modified to "lithon" (com- mudstone. Dolomite grains are equant and untwinned. Inhomogencities pare Marshak, 1983). within laminations include relict pellets, intraclasts, and irregular fenestrae. Several cleavage classification schemes have been proposed, but none Randomly oriented, <5-/nm-sized illite and chlorite grains comprise 1 of these is entirely appropriate for the cleaved dolomites of the Elbrook to 30 wt% of the dolomite layers (Fig. 4a). Fine-grained quartz silt is Formation. The classification in Figure 3 is a modification of those pre- always present in trace amounts and may constitute as much as 10% of the sented by Alvarez and others (1978), Powell (1979), Borradaile and dolomite by volume (avg = l%-2%). Pyrite content is approximately 3% others (1982), and Marshak (1983) and is based on spacing and morphol- (range is l%-8%) by volume. Calcite, found in only a few samples, is < 1% ogy of cleavage domains. Cleavage domain spacing can be classified as by volume. In general, with an increase in clay content, there is a deciease Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/6/778/3445197/i0016-7606-97-6-778.pdf