Prodip K. Dutta

Alluvial Sandstone Composition and Paleoclimate, I. Framework Mineralogy

ABSTRACT Systematic variations in compositional maturity of first-cycle fluvial sandstones are found in the Cutler Formation (Permian) and Fountain Formation (Permian-Pennsylvanian) in Colorado and in the Gondwana Supergroup (Permian-Triassic) of Peninsular India. These variations reflect changing climate during deposition. Climate is considered to be the critical factor affecting maturity because source rocks (granite, granite gneiss) and tectono-environmental setting (alluvial systems associated with basement-cored block uplifts) were similar and remained relatively unchanged throughout deposition of all three of the units. The roughly 3,200-m-thick Gondwana succession, consisting of five sandstone petrofacies, is characterized by the following sequence of change (oldest to youngest) in compositional maturity expressed in Q/F/R percentages: 54:42:4 88:10:2 62:34:4 82:16:2 99:1:0. This sequence is a function of the changing paleoclimate (glacial arid temperate humid warm semiarid warm semihumid warm humid) associated with the overall global limatic change and the changing latitudinal location of India as it moved from a location close to the South Pole northward during the interval of time represented by Gondwana deposition. Lithologic and fossil evidence indicates that, during deposition of the Fountain Formation, climate gradually changed from relatively wet and warm to at least semiarid and warm. This climatic change is the main factor responsible for the difference in compositional maturity of the older Fountain sandstone (Q:F:R-65:26:9) relative to the younger (Q:F:R-54:36:10). Cutler sandstone was deposited in an arid climate and has maximum compositional immaturity (Q:F:R-49:44:7). A log/log plot of the ratio of total quartz to total feldspar plus rock fragments against the ratio of total polycrystalline quartz to to al feldspar plus rock fragments is a sensitive discriminator of first-cycle sandstones with differing climatic heritage. Bulk chemical composition data support interpretation of climate from framework mineralogy, but by themselves are not sensitive enough to be unequivocal indicators. The same is true for the ZTR index and observations of degree and abundance of solution pits/embayments on detrital quartz. Results of this study support the conclusion of earlier empirical and theoretical analyses which suggest that the optimum conditions for the production and preservation of a distinctive climatic signature on sand composition are met in extensional plate-tectonic settings. Such settings are characterized by coarse-grained crystalline parent rocks, short transport distances in low-order streams, deposition in nonroutine environments, and shallow-burial diagenesis. However, even in such optimum settings, only rarely will climate be a more important determinant of sandstone composition than the tectonic setting itself.

Alluvial sandstone composition and paleoclimate; II, Authigenic mineralogy

ABSTRACT Various empirical and theoretical arguments suggest that early formed cement in sandstone is a product of meteoric water whose chemistry is controlled by climate. The nature and distribution of early silicate cement in Gondwana, Fountain, and Cutler sandstone support this supposition. Sandstones from all three units have experienced a broadly similar diagenetic history characterized by two stages of authigenesis. Stage I was typified by neomorphic development of kaolinite, chlorite, smectite, quartz, and, additionally in the Cutler Formation, laumontite. Stage II was dominated by replacement reactions involving the production mostly of illite, iron oxide, carbonate minerals, and, mostly in the Gondwana sandstone, a second generation of quartz. Thermodynamic relations, hydrologic constraints, and radiometric dating of the late-stage illite indicate that early authigenesis took place within a few million years of deposition at burial depths measured in hundreds of meters. Consequently, the nature of the silicate cement was a function of the groundwater chemistry, as controlled by climate. During times of relative aridity, ionic concentration of the groundwater was high, and high proportions of smectite, chlorite, and, more rarely, laumontite formed. Such was the case in Petrofacies I, IV, V of the Gondwana Supergroup (for a definition of Gondwana petrofacies see part I of this study) in the Cutler Fm., and in the upper 150 m of the Fountain Fm. During times of high precipitation, pore water was dilute, which promoted authigen c formation of kaolinite and quartz. This is represented by sandstones in petrofacies II and VI of the Gondwana and by sandstones in the lower 200 m of the Fountain Fm. Illite is especially abundant as a pseudomorphic replacement of kaolinite in the older Gondwana sandstones. Illitization apparently began at burial depths of about 1,600 m at a temperature of about 75°C. Illite is not common in the Cutler Formation and is present only in subordinate amounts in the Fountain Formation from the study area. The 18O values of early clay cement in the Gondwana sandstones range systematically from base to top from 5.00 to 13.20. The systematic variation in 18O value of Gondwana clay reflects the gradual migratory drift of the Gondwana harm toward lower latitudes through time. Average 18O values for Cutler cement (13.23) and Fountain cement (19.82) are greater than the analogous values for Gondwana cements. This is consistent with the lower-latitude setting of deposition for the Cutler and Fountain relative to the latitudinal location of the Gondwana basins. However, the values are anomalously lower than what have been observed in modern-day neoformed clays in weathering profiles from low latitudes.

Gondwana Lithostratigraphy of Peninsular India

Abstract The controversy regarding the stratigraphic interpretation of the Indian Gondwana succession in the Peninsular Basins has continued for more than a century, with the exception of the Damodar and Satpura Basins. While the stratigraphic reconstruction in Damodar and Satpur was based on the order of superposition of strata, elsewhere the relative ages were based on paleontological evidence that disregarded various physical criteria including the order of superposition. Overlying the Precambrian crystalline basement, the Gondwana succession of late Permian to early Jurassic age can be classified into four sedimentary facies; a glaciogenic facies (Facies A), a coal-bearing facies (Facies B), a red shale-sandstone facies (Facies C) and a hill-forming coarse sandstone-conglomeratic facies (Facies D). The stratigraphic order of these four facies in ascending order from Facies A at the bottom to Facies D at the top is well established in the Damodar and Satpura regions. In both areas, Facies D overlies the underlying older sediments as well as the crystalline basement with a pronounced unconformity. However, in the Pranhita-Godavari and Son Basins (and elsewhere) the stratigraphic relations between Facies B, Facies C and Facies D are controversial. One such enigma is where the sequence of Facies C and Facies D are repeated several times in the geological column. This interpretation contrasts with the supposed genesis of climatically controlled lithic fill in Gondwana Basins. Overturning the standard stratigraphic relation between Facies C and Facies D was based on fragmentary fossil evidence where the Lower Gondwana flora seems to be present in Upper Gondwana rocks. Turning the stratigraphy upside down was done without establishing the order of superposition between strata. Throughout the 20 th century, most stratigraphers working in the Pranhita-Godavari and Son Basins worked within a fossil-based stratigraphic paradigm. This resulted in still more confusion, the magnitude of which can be appreciated as one browses through various interpretations presented in innumerable publications. In this paper, we attempt to offer a lithostratigraphic interpretation of the Peninsular Gondwana Basins based only on physical criteria such as lithological association, their petrographical characteristics and the most fundamental tool of stratigraphy, “the order of superposition”.

In search of the origin of cement in siliciclastic sandstones: An isotopic approach

Abstract In siliciclastic sediments only a minor amount of cement is internally derived. This implies that an external source has to be sought in sandstone diagenesis. Controversy exists about the external source of cement and the nature of the aqueous solution, viz. meteoric, hydrothermal and marine, and/or dewatering of shale. Isotope geochemistry may prove useful in solving this controversy. The oxygen isotopic compositions of neoformed clay minerals are related to the isotopic composition of the coexisting aqueous solution. Aqueous solutions of diverse origin such as meteoric, hydrothermal and marine, and dewatering of shale, in general, bear characteristic isotopic signatures. Thus, the δ 18 O-values of authigenic clay minerals may be used to identify the nature of the original aqueous solution, provided the temperature of the environment is known and the extent and the nature of isotopic exchange between the minerals and fluids following crystallization can be evaluated. Oxygen isotopic compositions of early authigenic clay minerals in Permo-Triassic Gondwana sandstones of Peninsular India have been used to infer the cement source. Sandstones of Sakmarian age have an average δ 8 O-value of +5.0%o, whereas a relatively high average δ 8 O-value of +13.2%o is observed in sandstones of Rhaetic age. δ 18 O-values of sandstones ranging in age between Sakmarian and Rhaetic have values between +5.0 and +13.2% 0 . This gradual increase of δ 18 0-values with decreasing age shows a strong correlation with the changing latitudinal location of the sample site from a position around 60°S during Sakmarian time to a position around 38°S during Rhaetic time. The changing pattern of oxygen isotopic composition of suthigenic clay has been interpreted as a result of corresponding change in isotopic composition of meteoric water due to the northerly drift of the Indian plate during Gondwana sedimentation. This suggests that the aqueous solution, involved in the early diagenesis of Gondwana sandstones, is of meteoric origin.

Provenance of chert in the Permo-Triassic Sydney Basin, Australia: oxygen isotopic evidence

Abstract Detrital chert in chert-rich Permo-Triassic sandstones in the Sydney Basin, Australia, has been previously thought to have been derived from marine bedded chert in the New England Fold Belt (NEFB). This marine bedded chert is associated with volcanic, sedimentary, and metasedimentary rocks. Though volcanic detritus is common in these chert-rich sandstones, sedimentary and metasedimentary rock fragments are rare. This rarity raises the question of whether the source of chert grains in these sandstones was, indeed, the marine bedded chert in the NEFB or whether there was an alternative source. Without confirming the origin of chert, the provenance interpretation seems equivocal. Since the conventional petrographic technique proved to be inadequate in determining the origin of chert, it is necessary to use a technique that can discriminate chert of one type from another. The oxygen isotopic composition of chert from different environments shows characteristic δ 18 O values and, therefore, is used in this study to differentiate chert based on their isotopic signatures. The oxygen isotopic composition of chert grains in Permo-Triassic sandstones in the Sydney Basin was determined and the δ 18 O values range between 12.7 and 20.8%c (SMOW). On the other hand, the isotopic composition of metamorphosed marine chert in the NEFB ranges between 23.0 and 25.2%. Texturally, detrital chert shows volcanic relicts while the bedded marine cherts show foliated texture. Both isotopic composition and texture are consistent with the interpretation that the primary source of the detrital chert was not from the NEFB but from an alternative source: chemically weathered volcanic rock.