Cleavages developed in mudstone during diagenesis and deformation: an example from the Carboniferous (Tournaisian), southeastern New Brunswick, Canada.
The Weldon Formation of the Lower Carboniferous (Tournaisian) Sussex Group of southeastern New Brunswick consists of red mudstone intervals interbedded with red siltstone and fine-grained sandstone. Some of the mudstone intervals exceed 50 m in thickness and are deformed in such a way as to render bedding ambiguous. At two localities south of Moncton, these thick mudstone intervals carry two fracture cleavages that are almost perpendicular to each other, and perpendicular to bedding. In the Weldon Creek section these cleavages are particularly well preserved, and a clear timing relationship can be established. The second cleavage crenulates and transposes the first, which typically carries a slicken lineation where lithons are deformed by crenulation. Thin section and scanning electron microscope images reveal that these cleavages are defined by microveins of calcite, gypsum, and montmorillonitic clays with hematite and vivianite, and that there is some evidence for silica dissolution. At Belliveau Village, these two cleavages are largely disrupted by layer-parallel shear planes in the limb of the Belliveau syncline, indicating that they predated folding. The presence of primary diagenetic gypsum, primary carbonate cement in interbedded sandstone, and low equivalent vitrinite reflectance (%R) values in underlying oil shale (0.65-0.77) indicate that these cleavages formed below 100 [degrees]C during diagenesis rather than under anchizone conditions. Their presence reflects a bulk shortening parallel to bedding prior to fold development, and layer-parallel deformation during folding progressively destroyed them. These early cleavages are not precursors to slaty cleavage, and are essentially ephemeral features, though they could provide another anisotropy on which later cleavages might nucleate.
La formation de Weldon du Carbonifere inferieur (Tournaisien) du Groupe de Sussex, dans le sud-est du Nouveau-Brunswick se compose d'intervalles de mudstone dans des stratifications de siltite rouge et de gres a grain fin. Certains de ces intervalles de mudstone ont plus de 50 m d'epaisseur et presentent une deformation telle que le litage semble ambigu. Dans deux localites au sud de Moncton, ces intervalles epais de mudstone comportent deux schistosites de fracture se presentant pratiquement a la perpendiculaire, l'une par rapport a l'autre, et egalement a la perpendiculaire de la stratification. Dans la section du ruisseau Weldon, ces schistosites sont particulierement bien preservees et on peut en deduire tres nettement une correlation de periodicite. La deuxieme schistosite donne lieu a une crenulation reportee a partir de la premiere schistosite, laquelle comprend une lineation de glissement classique dom les lithons ont ete deformes par crenulation. Les images de lame mince et de MEB (microscope electronique a balayage) indiquem que ces schistosites se caracterisent par de petirs filonnets de calcite, de gypse, et d'argiles de montmorillonite assortis d'hematite et de vivianite, et on peut egalement observer des traces de dissolution de silice. Au village de Belliveau, ces deux schistosites sont alterees de maniere importante par des plans de cisaillement paralleles au litage dans le flanc du pli synclinal Belliveau, ce qui porte a croire que les schistosites sont anterieures a la formation du synclinal. La presence de gypse diagenetique primaire, de ciment carbonate primaire dans le gres intercalaire, et d'une reflectance de vitrinite de faible equivalence (%R) dans le schiste bitumineux sous-jacent (0.65-0.77) semble indiquer que la formation de ces schistosites s'est operee a une temperature inferieure a 100 [degrees]C au cours de la diagenese, plutot que pendant l'anchizone. Cette presence rend compte d'un raccourcissement des matieres meubles parallelement au litage, survenu avant la formation du synclinal, et d'une deformation parallele au litage pendant la formation du synclinal, ce phenomene ayant entraine leur destruction graduelle. Ces premieres schistosites ne sont pas des indices d'un clivage ardoisier, caril s'agit pour l'essentiel de formations de nature ephemere, qui pourraient neanmoins produire une autre anisotropie susceptible de donner lieu a la nucleation d'autres schistosites.
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INTRODUCTION AND GEOLOGICAL BACKGROUND
Fabrics defined by the alignment of clay particles are widely reported from mudstones, and generally attributed, if they are bedding-parallel or sub-parallel, to compaction during dewatering and diagenesis (see review in Kisch 1991 and papers by Oertel and Curtis 1972, Moon 1972, Krizek et al. 1975, Curtis et al. 1980, and Maltman 1981). Discordant fabrics are generally attributed to early tectonic deformation and considered to represent incipient cleavage development under conditions transitional from high-temperature diagenesis into anchizone and greenschist-facies metamorphism, and in such cases, fabric geometry can be related to finite strain (e.g., Dunoyer de Segonzac 1966; Kubler 1967a, 1967b; Pique 1975,1982; Weber 1976; Siddans 1977; Dandois 1981; Kisch 1989; Glasmacher et al. 2004). What is described in this paper is a pair of cleavages, highly discordant to bedding, in a mudstone where diagenetic history and thermal maturation data (vitrinite reflectance and spore coloration) indicate conditions that were well below the anchizone-diagenesis transition. The geometric relationship of these cleavages to bedding and regional structures (folds and thrust faults) suggests that they formed during a very early phase of bedding-parallel shortening, and were progressively obliterated by layer-parallel slip and shear. Not only were these cleavages formed under exceptionally low-grade conditions, but they are also ephemeral features of these mudstones, largely destroyed by subsequent deformation.
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The Lower Carboniferous sedimentary rocks in the area south of Moncton, New Brunswick, are contained in a series of northeast-southwest anticlines and synclines in the Hillsborough area, that are part of a fold-thrust belt along the southern edge of the Moncton sub-basin (Figs 1, 2; Park and St. Peter 2005; Park et al. in press). These features all lie beneath the unconformity at the base of the late Tournaisian to Visean Hillsborough Formation (Park and St. Peter 2005; Park et al. in press; St. Peter and Johnson 2009). The pre-Visean (Fammenian-Tournaisian) rocks below the Hillsborough Formation are included in two stratigraphic units: Horton Group and Sussex Group. The Horton Group is subdivided into a basal Memramcook Formation (absent locally) and an overlying Albert Formation. An unconformity separates the Horton Group from the overlying Sussex Group. The Sussex Group is in turn subdivided into a conglomeratic Round Hill Formation, a red mudstone-dominated Weldon Formation, and a grey-red shale - evaporite Gautreau Formation. The three formations are partly lateral equivalent, but in the area of this study the Round Hill Formation is overlain by, and interbedded with, the Gautreau Formation, and both are overlain by the Weldon Formation (Park and St. Peter 2005; Park et al. in press; St. Peter and Johnson 2009).
The Weldon Formation is a thick succession of mudstone interbedded with red siltstone and red to brown sandstone, for which a thickness of at least 300 m is proven from boreholes. Mudstone is the dominant lithology, and locally comprises intervals in excess of 50 m with no siltstone or sandstone interbeds and with very poorly defined bedding. Gypsum rosettes are present in much of this mudstone, and iron-reduction spots and fracture-related iron-reduction zones are ubiquitous. Siltstone and sandstone beds are generally less than 10 cm thick, and are typically laminated or cross-laminated on a sub-centimetre scale. Channels, scours and rippled surfaces are common, reflecting the floodplain to lake-margin depositional environment that is inferred for these rocks (see, for instance, St. Peter 2006).
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CLEAVAGES IN MUDSTONE OF THE WELDON FORMATION
Early cleavages are best preserved in the mudstone that is exposed in a large section along Weldon Creek near Isaiah Corner (Albert County). The progressive destruction of early cleavages is best observed in the cliff section and tidal platform along the Petitcodiac River estuary at Belliveau Village (Westmorland County). These two localities are described in detail below.
Weldon Creek, Isaiah Corner
A succession of at least 50 m of red mudstone with very few sandstone/siltstone interbeds is exposed in 150 m of continuous section along the southern bank of Weldon Creek, 1 km south of Isaiah Corner. The bedding, as defined by the sparse interbeds, dips south at approximately 30[degrees] (Fig. 3a). According to McLeod (1980) these mudstones lie between the main Saint-Joseph Fault and a southern splay, and in the northwestern limb of the Belliveau syncline, although neither is exposed in the creek section (Fig. 1). The mudstones are very fissile, breaking into a steeply plunging 'pencil' structure that is formed by the intersection of two steeply dipping cleavages (Fig. 3b). The definition of 'pencil structure' used here differs from the more restricted 'prolate strain'-defined pencil structure of Ramsay (1981) and Kligfield et al. (1983). It more closely conforms to the 'intersection' structure of Reks and Gray (1982) and Ferrill (1989). The development of the second cleavage is variable, and lithons several metres wide define zones in which the first cleavage is preserved with minimal development of the second (Figs. 3a-e). The transition from these lithons into the zones of intense and well-defined pencil structures affords evidence of overprinting and the transposition of the first cleavage ([S.sub.a]) by the second cleavage ([S.sub.b]) (Figs. 3b-e). Both cleavages can be traced continuously through the full 50+ m mudstone interval exposed in Weldon Creek, demonstrating that they are not paleosol structures or 'peds' (Retallack 1988) developed during deposition of the Weldon Formation. Peds are seen within the Weldon Formation but do not extend more than a few metres below well-defined paleosol surfaces.
The first cleavage ([S.sub.a]) is a spaced, smooth cleavage (Kisch 1991), with lithons ranging in width between 0.25 and 1.0 cm. The fractures are commonly defined by growths of illite, hematite (some specularite), and vivianite (hydrated iron phosphate--[Fe.sub.3] (P[O.sub.4)2] x 8[H.sub.2]O) giving the surface a 'metallic' sheen and bluish-grey appearance. Within the macro-lithons, in the absence of penetrative [S.sub.b], this first cleavage surface is typically unadorned, but within the macro-lithons and near their margins, [S.sub.a] carries a slickenside lineation ([L.sub.a]) that is especially pronounced when hematite and/or vivianite are present (Fig. 3e). In thin section, the expression of [S.sub.a] by aligned fine-grained phyllosilicates is very weak, and unless hematite and/or vivianite are present the septae are diffuse (Figs. 4a, b). In some cases microveins are present, and there is a distinct zone up to 0.1 mm on either side of such veins in which quartz clasts are rare, suggesting that some silica dissolution has occurred in relationship to these features. These narrow zones are also markedly less reddened than the mudstone in general. In its undisturbed state, [S.sub.a] is nearly vertical and strikes west-southwest. The slickenside lineation ([L.sub.a]) is close to horizontal.
The second cleavage ([S.sub.b]) has a close superficial resemblance to [S.sub.a] in being a spaced, smooth fracture cleavage with lithons ranging in width between 0.25 and 1.0 cm. The cleavage itself has a very clear expression in thin section. It is defined by aligned phyllosilicates along the immediate margins of septae, and re-inforced locally by gypsum and calcite veinlets (Fig. 4c). Some hematite is present in the septae, but not as much as that which typically adorns some of the septae of [S.sub.a]. Some evidence of silica dissolution is also present in selvages along the [S.sub.b] cleavage (Fig. 4c).
Overprinting relationships between [S.sub.b] and [S.sub.a] commonly take on two forms, depending on the spacing of the [S.sub.b] septae. Where [S.sub.b] septae are widely spaced, or form clusters of closely spaced septae separated by zones with few [S.sub.b] septae, the Sa cleavage is asymmetrically folded, with wavelength approximating twice the amplitude. One limb typically shows stronger [S.sub.b] development than the other, and in the limb that is more closely aligned to the [S.sub.b] direction, [S.sub.a] is transposed (Figs. 3b, c). On the other limb, crenulation of [S.sub.a] by [S.sub.b] is evident. The fold axes plunge parallel to the intersection lineation (Lab), forming the 'pencil structure' noted above.
Where the development Of [S.sub.b] is more uniformly intense, [S.sub.a] is preserved only within the [S.sub.b] lithons, forming a pronouncedly curved surface (turning into parallelism with [S.sub.b]), and showing the strongest development of the slickenside lineation [L.sub.s] (Fig. 3e). There is an evident correlation between the degree of curvature on the [S.sub.a] relics and the intensity of [L.sub.s].
The relationship to bedding is a striking feature of these cleavages in the Weldon Creek section. At the eastern end of the outcrops in Weldon Creek, siltstone-sandstone interbeds are more numerous than elsewhere in this section. Both [S.sub.a] and [S.sub.b] are almost perpendicular to bedding, and yet there are very few places in which bedding shows any obvious displacement along either cleavage (Fig. 3f). Indeed, the first real disruption of bedding noted in this section is along a set of shear planes that seem almost parallel to [S.sub.b] on the outcrop surface, but that differ from it by about 30[degrees] in strike (Fig. 3f). These same shear planes disrupt the [S.sub.b] cleavage (offset by less than 2 cm) and are clearly younger features.
Stereographic representation of this geometry makes it clear that the cleavages are close to perpendicular to each other and to bedding (Fig. 6a). The slickenside lineation ([L.sub.s]) on the [S.sub.a] cleavage lies close to the strike of bedding, and crucially, when this lineation shows signs of having been rotated during the transposition of [S.sub.a] into [S.sub.b], the rotation is in a plane that is almost parallel to bedding (Fig. 6b). This implies that any movement across these cleavage planes, and indeed any associated deformation, involved only bedding-parallel motion.
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Morphology of these cleavages is very similar to those described in the Mabou Group or slightly younger (Visean or possibly Serpukhovian) mudstone-siltstone in southwestern New Brunswick (Stringer and Lajtai 1979: note that these authors considered the rocks in question to be Triassic, but subsequent workers have ascribed them to the Carboniferous following the discovery of Windsor Group at the bottom of the succession, see Park et al. 1994 and references therein).
Belliveau Village--Petitcodiac River section
Some 150 m of the Weldon Formation are exposed along just over a kilometre of cliff and foreshore outcrop at Belliveau Village on the eastern shore of the Petitcodiac River estuary (Fig. 1). The southern third of the section is a red mudstone with a few interbeds of sandstone/siltstone, whereas the rest of the section contains roughly equal amounts of mudstone and sandstone-siltstone units. This latter part of the section defines the closure of the Belliveau syncline (Fig. 6a), whereas bedding in the southernmost third of the section is ambiguous.
The part of the section of most interest lies in the southern (northward-dipping) limb of the syncline around a large headland where the first sandstone-siltstone interbeds appear above the lower, uniform red mudstone. South of this headland the mudstone outcrop is broken up into polygonal domains (polygons on outcrop surfaces that are planar--polyhedra in the third dimension) by at least four sets of shear planes, some of which are associated with greenish iron-reduction zones (Figs. 5a-d). Within the polygons, there is a sporadic preservation of two cleavages ([S.sub.a] and [S.sub.b]) with intersection that create a pencil structure--[L.sup.a.sub.b]. Both are fracture cleavages with lithons 0.25-1.5 cm wide, as at Weldon Creek, but without the specularite-vivianite that is seen there. Where the first siltstone-sandstone units appear in the section, bedding is disrupted by the same shear planes that define the polygons in the mudstone (Figs 5e, 4f). When vestiges of the pencil structure [L.sup.a.sub.b] and the two cleavages ([S.sub.a] and [S.sub.b]) are present and where bedding is defined by siltstone-sandstone layers, no offsets are visible congruent with these cleavage planes. Post-[S.sub.b] shear planes disrupt both bedding and the earlier cleavages (Figs. 5a-d).
In contrast to the Weldon Creek section, the section at Belliveau Village is dominated by post-[S.sub.b] shear planes (labelled X, Y, Z, A in Figs. 5a-d). These shear planes are plentiful enough to consider their relationship to the relatively well-defined and exposed Belliveau syncline. The [S.sub.a], [S.sub.b] and [L.sup.a.sub.b] relics show little coherence in their geometry in stereographic projections, which is consistent with the disruption that is seen in outcrop (Fig. 6b). Evidently much rotation and shearing were involved in the creation of the polygonal pattern in the mudstones, and the orientation data set for these features is too small to define a broader pattern. The shear planes are a different matter, especially those which are cut by, and therefore predate, those shear planes associated with iron reduction. Though there is much scatter, two sets (A and B in the stereonets, Fig. 6b) appear to define a conjugate pair symmetrically oblique to the axial plane of the Belliveau syncline (Fig. 6b). They seem to be symmetrically disposed about the a-c plane of the syncline--see Hobbs et al. (1976, Fig. 7.4), and are comparable the R' and R" joints of Price and Cosgrove (1990, Fig. 14.20). Though the intersection of this conjugate pair does not coincide with the pole to bedding, it is close to this point. Movement evident on some of these joints as indicated by slickensides, is evidently more complex than that of a simple conjugate pair, yet it is movement on these joints, now shear planes (Y in Figs 5a-d), that created the first substantial disruption of bedding. The acute angle of the conjugate shear planes suggests a response to north-south shortening that also created the Belliveau syncline and the Saint-Joseph Fault (when it moved as a thrust). This implies that disruption of bedding is created by displacement on structures that have a consistent geometrical relationship to folding, and as a corollary, it also implies that the earlier pair of cleavages ([S.sub.a] and [S.sub.b]) predate that folding (or rotation of the limb on which they are located). The acute intersection of conjugate pairs of joints that are symmetrically disposed about the a-c planes of the Belliveau synform indicate shortening along a north-south direction (Fig. 6b). The scatter in the poles to cleavages implies more rotation (Fig. 6b), probably a consequent of layer-parallel shear in the rotating limb of the syncline. Both the Belliveau and Weldon synclines, to which these cleavages have a geometrical and genetic relationship, predate the unconformity below the Hillsborough Formation (see Fig. 1, also Park and St. Peter 2005, Park et al. in press).
It should also be noted that the later shear planes associated with iron-reduction features are also associated with hydrocarbon migration. No bitumen has been observed or reported in the Belliveau Village section, but similar iron-reduced shears and sandstones within the Weldon and Gautreau formations at Boudreau Village, 1.5 km south of this location, carry pyrite and bitumen (Park and St. Peter 2005). This indicates that hydrocarbon migration from the underlying Albert Formation (Horton Group) post-dated cleavage formation and its initial disruption.
SCANNING ELECTRON MICROSCOPY OF CLEAVAGES
Scanning Electron Microscopy (SEM) was undertaken on samples from Weldon Creek. The mudstone is exceptionally difficult material to sample on account of its fissility, friability and the presence of vivianite, which is hygroscopic and decrepitates in moist air. Samples had to be diamond-sawn out of outcrop and encased in surgical plaster before removal, then oven-dried and impregnated with polyvinyl acetate prior to cutting in order to prevent complete disintegration. Optical examination of thin sections revealed a close association between veinlets and both the [S.sub.a] and [S.sub.b] cleavages (Figs. 4a-c), but aside from confirming the presence of hematite and vivianite along the [S.sub.a] and [S.sub.b] cleavages, iron-staining and extremely fine grain-size precluded more detailed analysis. SEM images reveal more detail of both fabrics, and confirm the presence of veinlets that reinforce diffuse clay mineral septae (Figs. 4d, e). Both deformed detrital illite grains, and authigenic clay grains with preferred orientation are present, defining both [S.sub.a] and [S.sub.b]. In both cases, veinlets of an irresolvable fine mixture of clay, gypsum, and calcite are present (Figs. 4d-f). Crosscutting relationships are also made apparent by the transposition of clay septae, where [S.sub.b]-related veinlets cut the [S.sub.a] cleavage (Fig. 4f). Qualitative data (EDAX spectra) imply that the detrital clays are largely illite (with negligible peaks for Ca, Na or Mg), whereas the authigenic orientated grains, and the material that is mixed with ultra-fine calcite, have a more montmorillonitic composition (Ca and Na peaks appear alongside K peaks in qualitative EDAX spectra). The presence of such clay with swelling properties may account for why the two cleavages form such good partings near weathered surfaces. SEM images did not reveal any clay mineral fringes on the detrital quartz or feldspar grains that are sporadically present in these mudstones.
SEM is the only microtextural analytic technique practically applicable to this material. Attempts to assess illite crystallinity by X-ray diffraction (XRD) proved ambiguous as the mudstones are still dominated by detrital clays. Difficulties in obtaining standard thin sections from such friable material precluded attempts to prepare wafers and apply transmission electron microscopy (TEM).
TIMING OF CLEAVAGE FORMATION
Timing of cleavage formation in these rocks relates to: (1) formation of the cleavages relative to the formation of folds, and (2) formation of cleavages in relation to the diagenetic history of these rocks. Three observations in the immediate environment of these cleavages pertain to the relationship with the diagenetic history. First, the mudstones contain gypsum rosettes with no trace of anhydrite (a development similar to that seen in the cleavage micro-veins), implying that conditions (especially temperature) have been consistent with gypsum stability ever since the rosettes (and micro-veins) formed. Second, the first evidence for reducing fluids percolating through the mudstones is iron-reduction haloes around shear planes that all cross-cut the two early cleavages and the conjugate pair of shear planes. There is no iron-reduction associated directly with either [S.sub.a] or [S.sub.b] (both the hematite and vivianite associated with [S.sub.a] and [S.sub.b] at Weldon Creek are oxidized phases). Elsewhere in the area these iron-reduction features are associated with hydrocarbon migration, implying a later stage of maturation-diagenesis. Third, the sandstone layers within the mudstone sections have their primary carbonate cement intact with no evidence of secondary porosity formation or silica overgrowth. These three characteristics imply that an early stage of diagenetic history is preserved in these rocks, and that the highest temperatures they have experienced lie in the range 60-100 [degrees]C. Depending on when the gypsum rosettes grew, the lower end of this range is more probable. This observation places severe constraints on depth of burial. It also places these rocks below the range for conditions consistent with 'anchimetamorphism' (see Kisch 1989, 1991).
Such conditions for maximum burial depth and thermal maturation do not rule out the formation of cleavage. Indeed, the appearance of a fissile fabric in claystone particularly is well documented (Kisch 1989, 1991 for reviews; see also Oertel and Curtis 1972; Curtis et al. 1980, Krizek et al. 1975; Maltman 1978, 1981). However, these early cleavage-like fabrics are produced by compaction and dewatering, and the resultant single fabric is always bedding-parallel, or close to being in this orientation. This situation is in marked contrast to the case described at Weldon Creek, where the two cleavages are both perpendicular to bedding. Furthermore, the fabrics that are created by compaction and dewatering are generally described as 'fissility' (a penetrative fabric) rather than as a spaced, domainal foliation (Moon 1972). There may well be a continuum between the two, but the end-members of shaly fissility and domainal cleavage are morphologically quite distinct.
Bedding-parallel fissility forms during compaction and may be associated with deformation as well (Curtis et al. 1980), but this deformation generally resolves to a shortening perpendicular to bedding. Deformation associated with [S.sub.a] and [S.sub.b] in these mudstones is quite distinct in this respect. Veinlets parallel to [S.sub.b], and also perpendicular to bedding, show no evidence of shortening along their lengths, strongly suggesting that these veinlets were emplaced after the compaction associated with dewatering. Similarly, neither cleavage is deformed in a fashion that could be attributed to compaction, placing another constraint on timing with respect to diagenesis. The most marked deformation features associated with these cleavages are the development of the slickenside lineation on [S.sub.a] ([L.sub.s]), and the subsequent rotation of [L.sub.s] and curvature Of [S.sub.a] when it was transposed into [S.sub.b], none of which can be resolved by simple loading with a maximum principal stress that would be perpendicular to bedding. Here the fabric geometry suggests a bulk shortening parallel to bedding.
In summary, two cleavages formed perpendicular to bedding in these mudstones after compaction and dewatering, but prior to folding-related deformation associated with the Weldon and Belliveau synclines. Indirect evidence suggests that both cleavages formed during the early stages of diagenesis (prior to iron-reducing fluid and hydrocarbon migration), probably at temperatures less than 100 [degrees]C in rocks that were never deeply buried. Geometry of the fabrics does imply a relationship to bulk shortening of the layers parallel to bedding, possibly an early response to the same shortening that generated the folds. Critically, both cleavages predated the folds themselves.
CONDITIONS OF CLEAVAGE FORMATION
The conditions under which the [S.sub.a] and [S.sub.b] cleavages formed are constrained by two sets of independent data from either the Weldon Formation itself, or from underlying oil shale of the Albert Formation. Diagenetic minerals in the red mudstone of the Weldon Formation include gypsum, occurring as veinlets within the second cleavage ([S.sub.b]), or as rosettes in the matrix with an ambiguous relationship to either cleavage ([S.sub.a] and [S.sub.b]). Petrographically, there is no evidence of the presence or former presence of anhydrite, indicating that these red mudstones have never been buried sufficiently since an early stage of diagenesis to pass through the gypsum--anhydrite transition. Given that this reaction involves a very large molar volume change, it is pressure sensitive, but occurs at around 50 [degrees]C at atmospheric pressure. This temperature will fall with increasing pressure through burial, placing a severe constraint on depth of maximum burial (certainly less than 1.5 km).
Direct temperature estimates are also available from studies of the oil shales in the Albert Formation (Chowdhury 1995). Equivalent % vitrinite reflectance (%R) data from around Weldon Creek and Belliveau Village give values between 0.65 and 0.75 (with a maximum of 0.77 from a borehole at the albertite mine site, Albert Mines, 2 km from the Weldon Creek site). Thermal Alteration Index (TAI) from spore coloration has also been determined for rocks at Albert Mines, with values averaging 2.00 (Chowdhury 1995; Macintosh and St. Peter in press). These data translate to a maximum temperature in the 70 to 90 [degrees]C range, and critically, the lower figures come from the top of the Albert Formation typically 100 to 150 m below the level of the exposed Weldon Formation in which these cleavages are observed. These data imply that maximum temperatures in the Weldon Formation of Weldon Creek and Belliveau Village were no higher than 50 [degrees]C--consistent with the observations regarding diagenetic gypsum as rosettes or micro-veins in the cleavages.
It is useful to compare these conditions to those generally associated with early cleavage formation, namely the higher temperature diagenetic anchizone regime transitional to prehnite-pumpellyite metamorphic facies (Dunoyer de Segonzac et al. 1966; Weber 1976, 1981; Piqud 1975, 1982; Siddans 1977; Dandois 1981; Kisch 1989, 1991, 1998; Gutierrez-Alonso and Nieto 1996; Glasmacher 2004). These authors described conditions well above %R of 1.0 (as high as 3.0) for conditions of early cleavage formation and incipient metamorphism in the anchizone.
Although they are interpreted as tectonic features, these two cleavages and the resultant pencil structure cannot be related to any measurable strain (cf., Reks and Gray 1982). This is due to a number of factors, not least of which is the absence of reliable strain markers in these red mudstones. Thin section and SEM examination of the mudstones has not revealed any pressure fringes on detrital grains, despite the abundance of quartz and feldspar grains in some samples. Veinlets parallel to the cleavages ([S.sub.b] mainly) show no evidence of shortening or elongation, though in the case Of [S.sub.a] there is some evidence of quartz dissolution along the cleavage septae. If silica from detrital grains was being mobilized during [S.sub.a] formation, then the absence of fringes around other detrital grains may indicate very low strain, or at least strain that is well below the threshold defined by Reks and Gray (1982), and certainly less than the 40-60% shortening generally cited for conditions of incipient slaty cleavage formation (Kisch 1989, 1991). Though there is overwhelming evidence in these rocks that strain cannot be constant volume, fluid-mediated mass transfer on a scale necessary to accommodate 40-60% shortening is highly unlikely.
Finally, though now somewhat scattered, the two cleavages may have originated as a conjugate pair. Unlike the disruptive shear planes seen in the Belliveau section, such a pair would have no obvious relationship to the axial plane of any recognized fold. As an element of shear rather than simple shortening is implied by this orientation, 'fracture' cleavage is the preferred term for these features, rather than 'slaty' cleavage or incipient 'slaty' cleavage.
DISCUSSION AND CONCLUSIONS
The presence of cleavages in the mudstones of the Weldon Formation is significant for a number of reasons.
1. Timing relationships demonstrate that these cleavages formed later than compaction-dewatering but very early in the diagenetic history of these rocks (prior to hydrocarbon migration). Their geometry suggests that they are related to a local deformation that affected these rocks shortly after deposition and by the end of the Tournaisian (prior to the unconformity beneath the overlying Hillsborough Formation; Park and St. Peter 2005; Park et al. in press). The demonstration of a tectonic relationship for a very early pair of cleavages here implies that mechanisms other than dewatering-compaction may be important elsewhere.
2. Relationships of these cleavages to folding are particularly revealing: the rotation of beds during folding was partly accommodated on the very shear planes responsible for the progressive obliteration of both early cleavages. Although related to the deformation that ultimately produced the folds (a bedding-parallel bulk shortening), these two cleavages predate the folds themselves. [S.sub.a] and [S.sub.b] formed as an early response to bedding-parallel bulk shortening, and were subsequently disrupted and progressively obliterated as folding began and bulk shortening gave way to layer-parallel shear and rotation.
3. Both cleavages ([S.sub.a] and [S.sub.b]) are essentially ephemeral features of these mudstones that were preserved locally where favourable factors prevailed. They were probably far more widespread than current preservation suggests. Several authors have indicated the importance of pre-existing fabrics in the nucleation of new cleavages (Williams 1972; Cosgrove 1976; Gray and Durney 1979; Maltman 1981; Craig et al. 1982; Ho et al. 1996), though this is tacitly assumed to be an early fissility, caused by compaction-dewatering. The presence of cleavages in the Weldon Formation mudstones that are not compaction-dewatering features, and yet still ephemeral (and not axial planar) suggests that fabrics other than compaction-dewatering related fissility may form earlier than folding, and that they too provide anisotropies that can he recycled as new cleavages nucleate in an evolving rock mass during deformation.
4. The pencil structure created by the intersection of [S.sub.a] and [S.sub.b] is also unusual (see review in Reks and Gray 1982) in so far as such a structure is generally considered to be an intersection of a cleavage or incipient cleavage with a primary shale fissility, therefore an [L.sup.1.sub.0] intersection lineation, close to, if not perfectly parallel to a fold axis. At neither locality in which this pencil structure is preserved is it bedding-parallel, or anything other than close to being perpendicular to bedding. It also bears little relationship to the geometry of the large-scale folds.
5. The ephemeral nature of [S.sub.a] and [S.sub.b] in the Weldon Formation is critical. They are preserved locally where conditions permit, but the evidence from the Belliveau Village section suggests that once folding began layer-parallel slip and shear obliterated these features. This is in marked contrast to most anchizone cleavages described in the literature (see Kisch 1991), which are regarded as the precursors to the slaty cleavage that is more fully developed in higher grade anchizonal and greenschist facies pelites. The early cleavages described here are fracture cleavages with evidence of shear, rather than true 'slaty' cleavages or their precursors. Other than providing another planar anisotropy at the scale of clay grains, on which subsequent cleavages may nucleate, the cleavages in the Weldon Formation mudstones cannot be considered precursors to anything seen at higher grades of recrystallization.
Field work related to this research was carried out as part of a contract with New Brunswick Department of Natural Resources, funding and the support of director Les Fyffe are gratefully acknowledged. Perry Clark and Jonathan Lafontaine assisted with field work, and Calvin Nash produced excellent thin sections from some truly intractable material. Douglas Hall assisted with the scanning electron microscopy. Discussions with David Keighley, Clint St. Peter, Paul Wilson, Joseph C. White, and Paul F. Williams, were especially valuable in formulating conclusions for which the author alone takes responsibility. John Waldron and Pierre Jutras are thanked for thorough and useful reviews.
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Editorial responsibility: Sandra M. Ba
ADRIAN F. PARK
Department of Geology, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada <firstname.lastname@example.org>
Date received: 09 June 2008 [paragraph] Date accepted: 17 December 2009
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