Petrography of gypsum-bearing facies of the Codó Formation ( Late Aptian ) , Northern Brazil

An original and detailed study focusing the petrography of evaporites from the Late Aptian deposits exposed in the eastern and southern São Luís-Grajaú Basin is presented herein, with the attempt of distinguishing between primary and secondary evaporites, and reconstructing their post-depositional evolution. Seven evaporites phases were recognized: 1. chevron gypsum; 2. nodular to lensoidal gypsum or anhydrite; 3. fibrous to acicular gypsum; 4. mosaic gypsum; 5. brecciated gypsum or gypsarenite; 6. pseudo-nodular anhydrite or gypsum; and 7. rosettes of gypsum. The three first phases of gypsum display petrographic characteristics that conform to a primary nature. The fibrous to acicular and mosaic gypsum were formed by replacement of primary gypsum, but their origin took place during the eodiagenesis, still under influence of the depositional setting. These gypsum morphologies are closely related to the laminated evaporites, serving to demonstrate that their formation was related to replacements that did not affect the primary sedimentary structures. The pseudo-nodular anhydrite or gypsum seems to have originated by mobilization of sulfate-rich fluids during burial, probably related to halokinesis. The rosettes of gypsum, which intercept all the other gypsum varieties, represent the latest phase of evaporite formation in the study area, resulting from either intrastratal waters or surface waters during weathering.


INTRODUCTION
Ancient evaporites are economically important as sources for salts (e.g., halite and potash) and metals (e.g., Cu, Zn, Au), and as indicators for structural traps of oil and gas (Warren 1999). These deposits are abundant in sedimentary basins located along the Brazilian continental passive margin, being particularly developed in the Aptian-Albian transition , Uesugui 1987. The evaporites of the Codó Formation in the São Luís-Grajaú Basin ( Fig. 1) are the only ones available for surface studies in the north equatorial Brazilian margin. Detailed facies analysis of fresh exposures in several open quarries located in the southern and eastern margins of this basin has provided elements to support a depositional system represented by a lacustrine or sabkha complex (Paz and ). In addition, the deposits exposed in these localities constitute an excellent opportunity to better understand the post-depositional processes that took place after primary evaporite formation. Such approach concerning to the Aptian Brazilian evaporites from the north equatorial Brazilian margin has not been conducted yet.
A preliminary field investigation showed that the evaporites from the Codó Formation include several morphologies, suggesting they might not be all primary in origin. Petrographic studies of evaporites are useful to help deciphering their origin (e.g., , Kerr and Thompson 1963, Holliday 1970, Arakel 1980, Aref 1998. This type of study allows determining the succession of events that took place following deposition (e.g., Arakel 1980, Morad et al. 2000, Pérez et al. 2002. Thus, this type of study applied to the Codó Formation might contribute to better define and/or reconsider the proposed lacustrine/sabkha system. Deciphering the post-depositional history of these evaporites might help to decide on their potential for strontium and sulphur isotopic studies attempting to test the hypothesis of a possible marine influence during sedimentation, as previously proposed elsewhere (e.g., Rossetti et al. 2000).

GEOLOGICAL SETTING
The Gondwana split up took place through several steps, culminating in the Aptian with the break up of African and South American continents (Góes and .
This process led to final establishment of several rift basins along the equatorial Brazilian margin, where the São Luís-Grajaú Basin is one of the largest, occupying 150,000 km 2 .
This basin has been interpreted as a unique structural feature formed by combination of pure shear stress and strike-slip deformation (Góes and . The main rifting developed in the Albian, when fault offsets reached up to 400 m (Figs. 2A,B), but initial fault displacement gave rise to a shallow and widespread basin where the Codó Formation was deposited in the late Aptian. This unit is defined at the top by a regionally correlatable unconformity marked by an irregular, erosion surface with associated paleosol (Paz and . Exposures of the Codó Formation include shallowing-upward deposits attributed to lacustrine or saline pan-sabkha depositional environments (Paz and Fig. 3). In the Codó area, where a lacustrine system dominates, the shallowing upward cycles consist of bituminous black shales, evaporites and, subordinately, calcimudstones attributed to central lake depositional environments. These deposits grade upward into gray to green shales and limestones (i.e., calcimudstone, laminated to massive peloidal wackestone to grainstone, and sparstone), related to intermediate lake INPE ePrint: sid.inpe.br/ePrint@80/2006/08.08.20.50 v1 2006 environments. The top of the shallowing-upward cycles comprises massive pelite, shale, limestone (e.g., intraclastic grainstone, ostracodal wackestone to grainstone, ooidal to pisoidal packstone, tufa) and rhythmite of limestone and shale. These lithologies display a variety of sedimentary features consistent with marginal lake deposition, such as paleosol, karstic surface, fenestrae, meteoric cement, vadose pisoid. In the Grajáu area, a saline pansabkha depositional environment was proposed , being represented by evaporite and locally tufa, gray-green shale, and calcimudstone.
The evaporite facies consists of laminated gypsum (formed by alternating darker and lighter couplets that are up to 15 cm thick), gypsarenite, and massive-macronodular gypsum, which record deposition in flat lying areas, intraformational reworking of previously deposited evaporites, and remobilization of salts during burial, respectively.
Laminated gypsum occurs in both of the study areas, dominating in the Grajaú area, while gypsarenite was observed only in the latter. Massive to macronodular gypsum is widespread in the Codó area, occurring mostly in the nucleus of evaporite lenses that are up to 5 m thick, which enclose chunks of black bituminous shales. In the Grajaú area, these deposits form diapiric features that are up to 6-8 m high at the outcrop scale. The contact between the massive-macrogranular gypsum and the laminated gypsum is gradational, except locally where the diapirs are well defined by sharp boundaries.

PETROGRAPHY OF THE EVAPORITES
The evaporite facies from the Codó Formation were characterized petrographically through the analysis of 86 thin sections distributed along four vertical sections (Fig. 4).
1. Chevron gypsum. This type of gypsum was recorded in both of the study areas, being much more widespread in sections located in the Grajaú area. The chevron gypsum, which represents the lighter component of the laminated gypsum facies, was described in a previous work, and it forms horizontal beds that are up to 10 cm thick of vertically aligned crystals. Under microscope, the chevron crystals form twin planes and superimposed growth faces with acute angles arranged as a zig-zag, perpendicularly to the crystal long axis (Fig. 6A). The chevron crystals usually grew up or are mantled by thin discontinuous layers of shales (mostly with smectites). Some thicker layers of chevron gypsum might display packages of crystals that slumped down into the muds, resulting in a series of segments with superimposed lower concave up shapes.
2. Nodular to lensoidal gypsum or anhydrite. This evaporite was recorded only in suburface, occurring in association with shales. It consists of either nodules of gypsum or isolated lensoidal gypsum crystals that are parallel to bedding planes (Figs. 6B,C). The nodules are up to 5 mm long, and internally composed by a dark mixture of anhydrite and gypsum that form a massive cryptocrystalline framework.
3. Fibrous to acicular gypsum. This type of gypsum occurs invariably in association with chevron gypsum, likewise forming vertically oriented crystals similar to needles (Fig. 6D).
Although individual areas with dominance of fibrous and acicular gypsum were observed, in general these types of gypsum are intergraded, resulting in a closely interlaced framework. Noteworthy it is the frequent presence of relics and/or ghosts of the chevron gypsum within the acicular crystals.  figure 6C). Noteworthy is that the anhydrite clasts occur as spots intergraded with alabastrine and fibrous gypsum.

Rosettes of gypsum.
Rosettes of amber-colored gypsum occur disperse within the evaporites of the Codó area, being restricted to the massive to macronodular gypsum (Fig.   8C). The rosettes reach up to 5 cm of diameter, and consist of fibrous crystals arranged into a radial pattern. The rosettes unconformably cut into all the other gypsum described above.
Carbonates associated with gypsum or anhydrite consist of calcite cement that fill fractures in the brecciated gypsum or gypsarenite (Fig. 9B), even where this facies is obliterated by pseudo-nodular anhydrite or gypsum. The calcite also occurs as enlarged crystals that grew upon the gypsum from both sides of the fractures (Fig. 9C), in which case it forms caries in the mosaic gypsum. On the other hand, the alabastrine gypsum enters into relics of calcite, forming caries toward them (Fig. 9D). Rhombs of calcite after dolomite is locally present (Fig. 9E). Celestite might be also present in association with the carbonates, occurring as pyramidal crystals with etched edges.

PARAGENESIS
Combination of facies and petrographic characteristics revealed that the foregoing described evaporites show features supporting both primary and secondary formations. In addition, the petrographic study also showed that at least great part of the secondary gypsum might have been formed under influence of the depositional surface, thus reflecting the original brine characteristics.
Evidence for primary evaporite precipitation is particularly recorded in the laminated gypsum. We interpret that the chevron gypsum is most likely primary in origin because they make a pattern, consisting of vertically-aligned upward growing twin crystals that are similar, though in a much smaller scale, to many primary gypsum described in the literature (e.g., Schreiber and Kinsman 1975, Ciarapica et al. 1985, Hovorka 1987, Logan 1987, Handford 1991, Smoot and Lowenstein 1991, Warren 1999. In addition, the chevron gypsum is confined to the laminated gypsum, defining typical horizontal bedding planes. Important to mention also is that this gypsum forms pairs with the dark (nodular) gypsum, occurring in a very regular way throughout the sections, resulting in bundles that probably reflect seasonal brine fluctuations. Hence, the chevron gypsum would have formed under subaqueous conditions between the sediment surface and the brine due to sulphate saturation (Fig. 10A). Such feature is attributed to the progressive upward precipitation of salts on the floor of shallow (usually less than 2 m deep), supersatured brine pools (e.g., Logan 1987, Handford 1991, Smoot and Lowenstein 1991, Hovorka 1987, as suggested in a previous work (i.e, Rossetti et al. 2004).
Part of the light beds of the laminated gypsum is made of fibrous and acicular crystals. This gypsum morphology is similar to satin spar gypsum commonly recorded filling fractures in many publications (e.g., Richardson 1920, Shearman et al. 1972, Mossop and Shearman 1973, Stewart 1979, Gustavson et al. 1994. In this instance, however, the satin spar gypsum is not related to fractures but it records replacement of the chevron gypsum. This is indicated by gradation between these morphologies, and by the fact that the satin spar gypsum displays many relics and ghost of chevron gypsum. As replacement took place, the beds were slightly deformed, but still kept the primary horizontal bedding. Although clear evidence is lacking, the absence of other types of secondary gypsum in the light gypsum beds suggests that such replacement took place shortly after deposition, thus still under a strong influence of the primary brine.
Therefore, despite their authigenic nature, the satin spar gypsum is considered as a reflex of the primary evaporite bedding.
The satin spar morphology reflects crystal growth from very pure, supersaturated fluids, which favored extreme elongation parallel to the c-axis (Magee 1991). The close association of the acicular to fibrous gypsum with the chevron leads to suggest that formation fluids were driven from remobilization of sulphates from the chevron gypsum, whose formation naturally required high brine saturation. It is important to mention that neither of these authigenic processes were enough to destroy the primary lamination. This is taken as evidence to suggest that the formation of mosaic and acicular gypsum took place cyclically and close to the depositional surface, just shortly after formation of the individual bundles.
The darker laminated gypsum is attributed to displacive growth of crystals beneath brines derived from supersaturated pore fluids in the capillary and/or upper phreatic zone (Fig. 10B). This implies periods of descending ground waters and eventual exposure (e.g., Kerr and Thompson 1963, Southgate 1982, Warren 1999. Thus, the light and dark gypsum are taken as products of high and low water level, respectively, which, given the high frequency and symmetrical distribution along the vertical sections, are most likely due to seasonal fluctuations (Figs. 10C-E) The cyclic alternation of fibrous gypsum with dark gypsum containing nodular to lensoidal gypsum or anhydrite is also a reflex of changes in the depositional conditions. The formation of these morphologies is consistent with deposition taking place a few millimeter of the depositional surface by displacive intrasediment growth of crystals beneath the brine from supersaturated pore fluids in the capillary and/or upper phreatic zone (e.g., Kerr andThomson 1963, Warren 1999). As the brine level decreased, nodular to lensoidal gypsum grew displacively within the underlying sediments, here represented by shales. This is the most common habit of gypsum precipitated within the sediments, either in mudflats or other environments subjected to palustrine conditions (Magee 1991).
The presence of nodules composed by a dark mixture of anhydrite and gypsum forming a massive cryptocrystalline framework is consistent with this interpretation, being related to increased evaporation, probably due to climatic changes ).
Similar masses of mixing gypsum have been considered as the record of sulphate replacements during seasonal variations next to the depositional surface (p.e., Arakel 1980, Mees 1998, Mees and Stoops, 2003. The mosaic gypsum was formed by replacement of anhydrite, as indicated by the fact the abundant remains of this mineral within it. Mosaics of gypsum crystals with sutured contacts have been interpreted as a non-equilibrium texture of grain interpenetration at low temperature (cf. Voll 1960), probably reflecting formation under early diagenesis (e.g., Spencer and Lowenstein 1990). The occurrence of mosaic gypsum in association with the dark bundles, alternated with chevron to acicular gypsum within the laminated gypsum, might be taken as evidence of replacement soon after deposition.
Extreme low brine level with subaerial exposure resulted in gypsum fracturing due to desiccation, a process that gave rise to in situ brecciated gypsum and ultimately gypsarenite (Fig. 10E), the latter recording local reworking at the surface as recorded in other evaporite deposits (e.g., Sanz-Rubio et al. 1999, Schreiber andEl Tabakh 2000). The frequent upward gradation from the brecciated to the gypsarenite is consistent with this interpretation. The presence of mud cutans surrounding the clasts is attributed either to mechanical infiltration from downward flows or to the adhesion of residues on clast surfaces during reworking.
Following brecciation, there was a phase of widespread replacement, which gave rise to mosaics of large gypsum crystals affecting the dark beds of the laminated gypsum facies. This sequence of event is proposed based on the observation that large gypsum crystals encompass several clasts, attesting to pervasive cementation and/or replacement.
The presence of abundant relics of microcrystalline lath-like anhydrite within the mosaics attests to their origin following a period of anhydrite formation. Noteworthy is the fact that the mosaics were developed only in the darker bundles of the laminated gypsum, not affecting the lighter components. Instead, the chevron gypsum was replaced, as previously mentioned, by fibrous and acicular gypsum.
Fracturing at the surface created porous, which were cemented by calcite. Calcite also replaced gypsum near the fracture sides. Under subaerial conditions (i.e., vadose to freshwater phreatic), sulphate-undersaturated pore fluids dissolves gypsum and/or anhydrite and release Ca 2+ for precipitation of calcite as the CO 3  2 et al. 1983et al. , Kushnir 1985. The close association of calcite, dolomite and celestite in the studied evaporites of the Codó Formation suggests these processes as the most likely.
The pseudonodular anhydrite or gypsum is interpreted to represent a later phase of gypsum formation. First, this is suggested by its occurrence restricted to the massive to The spots with microcrystalline lath-like anhydrite that occur within the macronodular gypsum is probably a relic of one of the earliest phase of evaporite precipitation in the study area The paragenesis reconstituted with basis on petrographic relationships shows that these anhydrites form nodules that were in part replaced by the mosaic gypsum, as revealed by the fact that the later form concave embayments that enter into the anhrydrite. The margins of this gypsum are, in turn, ragged due to reaction with fibrous and alabastrine gypsum. These relationships support that the anhydrite nodules had an early development relative to all other gypsum phases present in the pseudonodular gypsum. If so, them it is possible that the anhydrite is temporally related to the anhydrite formed in the dark bundles of the laminated gypsum, which represents one of the earliest evaporite phases developed in the Codó Formation.
The final phase of evaporite formation is recorded by the rosettes of gypsum, as confirmed by the fact that these truncate all the other evaporite morphologies. Aggregates of large fibrous gypsum crystals forming rosettes similar to the ones of the study area have been attributed to the action of either intrastratal waters during burial or surficial waters during weathering (e.g., Shearman 1966, Holliday 1970, Warren 1999.

CONCLUSIONS
Despite the many varieties of evaporites recorded in the Codó Formation exposed in the eastern and southern margin of the Grajaú Basin, these deposits display many petrographic and faciological attributes that are consistent with an early formation, either by primary precipitation or early replacements when the sediments were still under influence of the depositional surface. These types of evaporites prevail particularly in the Grajaú area, where laminated gypsum facies dominates. In both areas, burial phases of gypsum seem to have developed only where gypsum was remobilized during halokinesis.
The lack of significant deep diagenetic modification of the Codó Formation is recorded also by studies focusing the limestones and shales interbedded with the evaporites. These data are consistent with the proposition that burial did not cause significant textural or mineralogical modification of the Codó Formation.
The petrographic studies presented here strongly motivate to undertake isotopic studies in the evaporites of the Codó Formation. The analysis should be carried out using samples from the laminated facies only, which preserves evaporites formed both primarily and shortly after deposition, providing information on original brine characteristics. On the other hand, the pseudo-nodular anhydrite or gypsum and the rosettes of gypsum should not be considered in this type of studies, as they represent later stages of gypsum formation due to deeper salt remobilization. Inasmuch, the spots of anhydrite within the pseudo-nodular anhydrite or gypsum should be also discarded in these analyses. Despite the interpretation that these evaporites might have formed contemporaneously to the early-formed nodular to lensoidal anhydrite, the microscopic studies revealed they were strongly replaced by limpid, alabastrine gypsum, being inappropriate for isotopic analysis that can be used for paleoenvironmental purposes.    Formation (see Fig. 1 for section location).    illustrating their composition of tiny, equant, lath-like crystals (an). Note that the edges of the nodules were replaced by fibrous (gf) or mosaic (gy) gypsum.