They are predominantly low-grade metamorphic rock fragments, but plutonic, sedimentary and gneissic fragments are common and trace quantities of volcanic rock fragments may be present. These and other framework grains are cemented by two distinct carbonates and a kaolinite. Textural relationships reveal a relatively simple cement stratigraphie sequence.
Provenance : the dominant factor in clay mineralogy of the upper Wilcox Formation
Ankerite is present in the earliest formed concretions and as scattered irregularly shaped crystals that replace framework grains or fill secondary porosity created by the dissolution of framework grains during late-stage diagenesis. Ferroan calcite forms a poikilotopic intermediate-stage cement. The late stage ankerite is not a volumetrically important cement. Chemical compositions of the glauconites in the pore system compared to those encapsulated by the early ankerite concretions and intermediate stage ferroan calcites record a systematic diagenesis of this clay component of the Wilcox Strickler and Ferrell, The average glauconite structural formulas reveal an increase in interlayer potassium and a progressive substitution of iron for octahedral aluminum.
Similarly, the ankeritic concretions should preserve the original composition of the detrital micas, chlorites and matrix clays; and diagenetic changes in them should be minimal. Minerals in the concretions should be good indicators of compositions at the time of deposition. II -Methods. Additional runs following. A semi-quantitative estimate of the clay-fraction mineralogy was obtained with the Schultz technique. Mica and chlorite polytypes were determined by X-ray powder diffraction methods using the d-spacings tabulated by Bailey and the procedure of Maxwell and Hower to estimate the relative abundances of the 1M and 2M mica varieties.
The partially defocused beam minimizes the potential loss of alkalis due to migration or volatilization but restricts the interference-free analytical spot to areas slightly larger than the beam diameter. Therefore, the electron microprobe can not resolve the compositions of finely interlayered materials. Amelia albite and Kakanui hornblende were used for X-ray intensity calibration standards.
All microprobe results were normalized to wt. Structural formulas for the layer silicates were calculated on the basis of 44 negative charges per unit cell assuming that all the Fe was ferric. Chlorite formulas were calculated on the basis of 56 negative charges with all Fe in the ferrous state.
III -Results. The average wt. Table 1 : Average weight percent of clay minerals in the Wilcox. No significant variation in mineralogy can be facies related. The most striking difference in the clay mineral composition of the Wilcox samples can be observed by comparing the average results for samples from early ankerite concretions and intermediate-stage, ferroan-calcite cemented areas illustrated in Figure 2. Diffraction peaks representing the 2M micas and la and lb chlorites were present in most samples, but detailed polytype distribution data could not be obtained because of the complexity of the assemblages.
Figure 2: Average clay mineral composition of tight, early ankerite concretions and ferroan calcite cemented sands of the first Wilcox sandstone littoral facies. The fine matrix clays are more Fe-rich and less aluminous than the muscovites and less Fe-rich than chlorites. The compositional variations are almost continuous and no correlation between composition and stage of cementation can be established. Table 2 summarizes the results of the chemical determinations and the structural formula calculations for the large muscovite and chlorite flakes in the Wilcox.
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The means and standard deviations of the analyses were calculated after excluding muscovites with less than 0. Table 2: Normalized chemical results and structural formulas of selected muscovite and chlorite flakes. Some of the muscovites contain paragonitic and other solid solution constituents while the chlorites may contain intermixed elements of layer silicates.
The mean octahedral occupancy for the muscovites assuming that all of the Fe is in the ferric state is 1. The sum of the octahedral cations for the chlorites assuming that all of the Fe is in the ferrous state is 4. Average structural formulas for chlorite and muscovite are listed below:. Most of the Ca reported for the chlorite is a result of contamination by adjacent carbonates. Figure 3: Aluminum and iron oxide ratios of matrix clays, muscovites, and chlorites.
Glauconite pellets are the only materials to demonstrate a recognizable diagenetic trend. Chlorite and muscovite analyses were obtained from fresh-appearing large flakes. Total iron expressed as Fe2 O3. Five of 1 1 fresh-appearing muscovite flakes have AI2O3 and Fe-oxide ratios that can be plotted Figure 4 within the low-grade metamorphic zones recognized by Miyashiro These muscovites are especially rich in AI.
The six remaining detrital muscovites are compositionally similar to micas derived from plutonic terranes Speer et al. The analyses of 14 chlorites from cemented areas Figure 5 of the sandstone exhibit Fe : Mg : AI ratios that lie between the compositions of Appalachian metamorphic chlorites reported by Zen and the diagenetic chlorites reported from the Wilcox of Texas by Boles and Franks Figure 4: Iron and aluminum oxide ratios of several detrital muscovites a's from sandstones of the upper Wilcox suggest their similarities to metamorphic muscovites.
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Low-grade metamorphic chlorites from the Appalachians Zen, and diagenetic chlorites from Wilcox sandstones of southwest Texas Boles and Franks, are shown for comparison. IV -Discussion. The chemical and mineralogical results reported above would be difficult to interpret without the knowledge provided by discrete grain analyses and the constraints indicated by comparing clay minerals and detrital flakes in concretions and open pore spaces.
The highly variable mineralogy of the clay fraction is a direct consequence of the clays being extracted from polygenetic sources such as the glauconite pellets, the intergranular matrix clay of assumed detrital origin, the pore-filling kaolinite and the fragments of large micas and chlorites obtained in the size separation procedure. The glauconite pellets reveal distinct diagenetic changes and help to demonstrate that the original clay composition is preserved in the ankerite concretions Strickler and Ferrell, The compositions of the muscovite and chlorite enclosed in the carbonate cements clearly indicate their similarity to the layer silicates of the Appalachian terrane.
They appear to have been transported with minimal alteration. The 2M polytype is especially common when detrital mica flakes are abundant.
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The sediments appear to preserve micas representative of the weathering sequence: crystalline 2M mica to partially expandable 1 M illite. The most common chlorite polytype is the lib variety, which Hayes considers to be the common polytype of metamorphic and igneous terranes.
All the polytype results strongly point to the detrital and weathering origin of the clays. A further evaluation of changes in the major element ratios is provided by plotting the chlorite results on a diagram Figure 6 similar to that used by Curtis et al. The blackened circles represent chlorites from Wilcox concretions and the open circles are from uncemented areas of Wilcox sandstones. The compositional range is greater in the cemented areas than in the cement free ones.
Many of the latter analyses plot within an area that is intermediate between the lib field and the swelling chlorite field. The larger open arrows represent the direction of compositional change in the Wilcox chlorites that is associated with weathering prior to the formation of the early cement. Weathering increases the Fe and AI ratio relative to Mg and the amount of interlayered potentially swelling material.
The direction of the compositional trend for Wilcox chlorites is the opposite of that proposed by Curtis et al. Our results neither confirm nor disprove that such a diagenetic trend exists among chlorites, rather they illustrate the viability of alternative interpretations. The Wilcox interpretation is based on compositions of chlorites preserved.
This study clearly identifies the importance of inherited characteristics on the clay mineral composition of the Wilcox. Figure 6: Composition of detrital chlorites from sandstones of the upper Wilcox Group. The weathering pathway open arrows causes the chlorite compositions to evolve in a direction opposite to the diagenetic pathway solid arrows proposed by Curtis et al. V -Provenance. Other investigators have recognized the southern Appalachians as important sources of Tertiary sediments in the Gulf Coast, especially during the Eocene Chen, ; Galloway, As early as , Grim postulated that the heavy mineral assemblages observed in Eocene Wilcox sands of Mississippi suggested the existence of an ancient "Appalachian River" flowing from the southern Appalachians to the Gulf of Mexico.
Isphording proposed that "an ancestral Tennessee River" may have discharged into the Gulf of Mexico in what is now eastern Louisiana. Based on heavy mineral assemblages found in the basal Eocene Claiborne of Texas, Todd and Folk proposed the metamorphic rocks in the southern Appalachians as a major sediment source. They also suggested that epeirogenic uplifting of the southern Appalachians occurred during the Eocene causing the deposition of sands rich in Appalachian sediments.
After studying heavy mineral assemblages and flow directions, Pryor and Potter and Pryor postulated that the Blue Ridge and Piedmont Plateau Provinces were also an important source of sediments in the upper Mississippi embayment. This investigation adds further evidence for the importance of the Appalachians as a source of sediments.
The eastern Louisiana sediments are very different from Wilcox sediments to the west. The latter were derived from sources that were rich in material with high smectite contents derived from volcanic sources.
Mineralogy, porosity and fluid control on thermal conductivity of sedimentary rocks
Murray , MacNeil , Winker and Loucks et al. The eastern Wilcox sediments derived from the Appalachians contained markedly higher quantities of illitic and chloritic materials and were not as affected by smectite diagenesis as the western sediments. Their clay assemblages are inherited ones modified by weathering in the source area and during transportation. Provenance is the dominant factor controlling the clay mineral composition. Provenance has exerted an important but different control on the clay minerals of the Wilcox of south Texas and southeastern Louisiana.
The diagenetic scheme proposed by Boles and Franks in which Mg and Fe released during the diagenesis of smectite lead to the precipitation of late stage ankerites cannot be applied in the eastern Gulf of Mexico. The highly smectitic clays derived from the western Interior Basin were never present. The early ankerities of southeastern Louisiana were so early that they must have derived their Fe and Mg from meteoric sources.
These products of. Upper Wilcox sediments in southeastern Louisiana accumulated in coastal. They were cemented by ankerite and ferroan carbonates during periods of periodic emergence or flushing by meteoric waters. They were never buried deeply enough to experience severe illitization. They inherited their illite and chlorite rich composition from the rocks and weathered materials of the southern Appalachians.
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The illites and chlorites preserved in early ankerite concretions of these Eocene sediments retain element ratios and polytype varieties indicative of derivation from metasediments and plutonic rock types. Variations among the Fe, Mg and AI ratios of chlorites in early concretions, intermediate stage ferroan calcite cemented zones and open pore spaces are a result of weathering. Samples with abundant 2M mica polytypes are associated with detrital muscovite flakes while most illitic clays are the 1M polytype. These characteristics and the absence of a depth-related clay mineral transformation suggest that provenance, rather than diagenesis, is the dominant factor in determining the clay mineralogy of these Gulf Coast sediments.
Stretching of units can result in their thinning. In fact, at one location within the Maria Fold and Thrust Belt , the entire sedimentary sequence of the Grand Canyon appears over a length of less than a meter. Rocks at the depth to be ductilely stretched are often also metamorphosed. These stretched rocks can also pinch into lenses, known as boudins , after the French word for "sausage" because of their visual similarity. Where rock units slide past one another, strike-slip faults develop in shallow regions, and become shear zones at deeper depths where the rocks deform ductilely.
The addition of new rock units, both depositionally and intrusively, often occurs during deformation. Faulting and other deformational processes result in the creation of topographic gradients, causing material on the rock unit that is increasing in elevation to be eroded by hillslopes and channels. These sediments are deposited on the rock unit that is going down.