recycled detrital quartz grains are sedimentary ... - www .gsapubs .org

Journal of Sedimentary Research, 2013, v. 83, 368–376 Research Article DOI: 10.2110/jsr.2013.28

RECYCLED DETRITAL QUARTZ GRAINS ARE SEDIMENTARY ROCK FRAGMENTS INDICATING UNCONFORMITIES: EXAMPLES FROM THE CHHATTISGARH SUPERGROUP, BASTAR CRATON, INDIA ABHIJIT BASU,1 JUERGEN SCHIEBER,1 SARBANI PATRANABIS-DEB,2

AND

PRATAP CHANDRA DHANG2

1

Department of Geological Sciences, Indiana University, 1005 East 10th Street, Bloomington, Indiana 47405, U.S.A. 2 Geological Studies Unit, Indian Statistical Institute, 203 Barrackpore Trunk Road, Kolkata 70108, India

ABSTRACT: Recycled detrital quartz grains are unequivocally identified if there are overgrowths on abraded overgrowths. We argue that the presence of such recycled grains even in small quantities in successions of quartz arenites indicates derivation from one or more sedimentary units from below an intrabasinal unconformity or an extrabasinal unroofing surface, as do detritus from characteristic rock fragments in polymict clastic rocks. In fact, detrital grains with inherited overgrowths are sedimentary rock fragments. We apply this petrographic criterion to evaluate the presumed unconformities that bound the Singhora, Chandarpur, Raipur, and the Kharsiya groups of the essentially Mesoproterozoic Chhattisgarh Supergroup in the Bastar craton – a critical Ur-craton vis a` vis Columbia and Rodinia. Optical examination of about 6000 grains in 12 thin sections reveal few recycled quartz grains in sandstone samples of the Chandarpur Group but about 4% recycled quartz grains with inherited abraded overgrowths in a sandstone unit of the Raipur Group. Characteristic rock fragments from a tuffaceous unit at the top of the Raipur Group are found in the basal sandstone of the Kharsyia Group. The data indicate that the presumed unconformities below the Kharsyia Group and the Raipur Group are real but the one above the Singhora Group cannot be confirmed on the basis of recycled grains. Published age spectra of detrital zircons from these sandstones are compatible with the above inferences drawn from detrital-quartz and rock-fragment petrography.

INTRODUCTION

Detrital quartz grains may be generated from igneous and metamorphic rocks, or, from sedimentary rocks. In general, the former are called first cycle quartz and the latter are considered to be ‘‘recycled,’’ noting that metasedimentary grains may be ambiguous (Suttner et al. 1981; Zuffa 1987). In this paper we use this criterion to define ‘‘recycled grain’’ and exclude the processes of multicycling or recycling, operating on unconsolidated sediments such as those on beaches, desert dunes, meander bars of large rivers, and other winnowing and chemical maturation processes that produce supermature quartz arenites ‘‘almost as remarkable as a pure single malt Scotch whiskey’’ (Dott 2003, p. 387). We use the concept of ‘‘recycled grain’’ as many have in the context of stratigraphic investigations (e.g., Eriksson et al. 2004). In siliciclastic arenites, detrital grains are commonly but not exclusively cemented by clay minerals, carbonate minerals, quartz, or a combination of minerals. Precipitated quartz cement, especially if early in the diagenetic history of the arenite, is commonly syntaxial and grows in optical continuity with the original quartz grain. This part of the cement is called quartz overgrowth (Adams et al. 1984, Plates 38, 39; Tucker 2001, Plate 4 c, d). Depending on the chemistry of pore fluids, overgrowths may occur on other detrital minerals. Clasts from quartz-cemented arenites may recycle into a subsequent arenite and may also be cemented with quartz and have new overgrowths on abraded rinds of such arenite grains. These recycled quartz grains are, therefore, sedimentary rock fragments. The importance of sedimentary rock fragments and recycled grains has not been lost on sedimentary petrologists (e.g., Blatt 1967; Blatt and Jones 1975; Ingersoll 1983; Zuffa 1987; Amorosi and Zuffa 2011). Blatt and Jones (1975) estimate that about 80% of detrital grains in a sandstone Published Online: April 2013 Copyright E 2013, SEPM (Society for Sedimentary Geology)

are recycled. Zuffa (1987; and in many other papers) have described criteria to identify coeval and intrabasinal carbonatic and volcanic grains, and even quartz grains with phosphatic cement. However, it appears that the importance of quartz grains with multiple abraded overgrowths has not been addressed adequately. Recycled quartz grains derived from older silica-cemented arenites may or may not fully retain their overgrowths during weathering, transport, deposition, and lithification. Diagenetic processes may also reduce and even obliterate inherited overgrowths. In general, all sedimentary processes tend to destroy the outer rinds of detrital grains (cf. Kuenen 1960). Thus, not all recycled quartz grains are likely to retain a memory of their immediate sedimentary heritage. Newly formed overgrowths on abraded syntaxial overgrowths are the definitive identifiers of recycled detrital quartz in a siliciclastic rock (Johnsson et al. 1988; Dott 2003, his fig. 5). Recycled quartz grains with abraded overgrowths are common in Holocene sands, especially in desert sands, produced dominantly by mechanical weathering of appropriate source rocks (e.g., Johnsson et al. 1988, their fig. 5; Critelli et al. 2003, their fig. 4G; Garzanti et al. 2003, their fig. 4D). Other types of recycled quartz grains, such as those from sandstones cemented with carbonate or clay minerals, must remain in the purgatory of ‘‘first-cycle quartz’’ by default because they may lack specific definitive identifiers. Because of the generally low solubility of silica, detrital quartz grains in arenites are rarely cemented all-around. Even in a 100% quartz-cemented arenite, only segments of peripheries of grains are cemented. Therefore, quartz clasts from such sandstones will only have partial coatings of overgrowths. Depending on the orientation

1527-1404/13/083-368/$03.00

JSR

RECYCLED QUARTZ AND UNCONFORMITIES

369

FIG. 1.—Simplified map of peninsular India showing major Proterozoic basins, cratons, and tectonic zones. The names of the basins are abbreviated: VI 5 Vindhyan; CH 5 Chhattisgarh; PG 5 Pranhita–Godavari Valley; I 5 Indravati; KBB 5 Kaldagi–Badami– Bhima; and CU 5 Cuddapah. The tectonic zones shown are: CITZ 5 Central Indian Tectonic Zone; EGMB 5 Eastern Ghats Mobile Belt; and SGB 5 Southern Granulite Belt. The small Singhora area under consideration is the appendage of the Chhattisgarh basin at the tip of the arrow. Modified from Bickford et al. (2011a).

of the plane of a thin section, some inherited overgrowths may not be visible under a microscope. A geometric analysis of the visibility of inherited overgrowths in thin sections is given by Sanderson (1984). We argue that recycled quartz grains, as defined by the above criterion, are indicators of erosional unconformities in sedimentary successions. The relative stratigraphic positions of the unconformities remain undefined: they may be intrabasinal, at the base of a basin, or an extrabasinal unroofing event. This petrographic criterion should be useful in discerning unconformities, especially in the absence of a difference of dip between successive units or lack of outcrops. The purpose of this paper is (1) to test if the use of recycled quartz to infer erosional unconformities is a viable technique, and (2) to use the evidence from recycled quartz grains in silica-cemented quartz arenites and test the erosional nature of presumed but controversial unconformities in the Mesoproterozoic Chhattisgarh Supergroup (ca. 1400–1000 Ma), Bastar craton, India (Fig. 1). GEOLOGICAL SETTING OF THE CONTROVERSY

The Chhattisgarh Succession The Chhattisgarh basin lies in the eastern end of the Bastar craton. A simplified lithostratigraphy of the Chhattisgarh succession (ca. 1400– 1000 Ma) is given in Table 1. The Chhattisgarh Supergroup lies on a Late Archean granitic basement including a small older greenstone enclave in

the east and an older narrow metamorphic belt (Kotri – Dongargarh orogen) mostly away from the southwestern and western margin of the Chhattisgarh basin (Ramakrishnan and Vaidyanadhan 2008). These metamorphosed belts include successions of metaquartzites, the protoliths of which were quartz-cemented quartz arenites. There are no unmetamorphosed sedimentary successions between the crystalline basement and the basal rocks of the Chhattisgarh Supergroup. Thus, with no sedimentary rock in the provenance of the Chhattisgarh succession, it is reasonable to assume that there was no significant external source of recycled sediments. Therefore, all recycled sedimentary rocks in the Chhattisgarh succession very likely have come from within the basin, with the caveat that extrabasinal fragments of metasedimentary rocks may be deceiving in appearance. The stratigraphic nomenclature of the Chhattisgarh Supergroup is in such disarray that we have chosen to retain only the group names of Das et al. (1992; designated a` la DPD) and Patranabis-Deb and Chaudhuri (2008; designated a` la SPD) in Table 1. The groups are bounded by presumed unconformities. The data that we consider pertain to the major sandstone units in each group; there are other minor sandstone beds and lenses in the formations that constitute the groups. In Table 1, the major sandstone units are identified with Roman numerals (I, II, III, and IV); their common names are given in italics while noting that there are severe nomenclatural disputes among different groups of stratigraphers. We have used *Sst I* and similar expressions without implying that they have any formal stratigraphic status.

370

A. BASU ET AL.

JSR

TABLE 1.— Simplified stratigraphic succession of the Chhattisgarh basin highlighting the presumed unconformities and the relevant sandstones. The ages are from Patranabis-Deb et al. (2007), Vaidyanadhan and Ramakrishnan (2008), Das, K. et al. (2009), Bickford et al. (2011a, b), and Das, P. et al. (2011).

The Controversy

(4)

Discussion of stratigraphic relations of Mesoproterozoic sedimentary rocks in a small area (, 1500 km2) near the village Singhora (N 21u 179, E 83u 19) in the southeastern end of the Chhattisgarh basin (, 33,000 km2; Fig. 2) has been a matter of definitions (e.g., Ball 1877; King 1885; Dutt 1964; Schnitzer 1971; Murti 1987), extrapolations, and revisions (e.g., Das et al. 1992, 2001, 2003, 2011; Patranabis-Deb and Chaudhuri 2008; Bickford 2011a; Dhang and Patranabis-Deb 2012). The debate has intensified recently (Saha et al. 2012). The mainstream in Indian geology (e.g., Naqvi 2005; Ramakrishnan and Vaidyanadhan 2008) embraces the stratigraphy of the basin as erected by the Geological Survey of India (Table 1 under ‘‘a la DPD’’; Das et al. 1992). Briefly, the points of the different opinions are as follows:

(5)

(1)

(2)

(3)

Saha et al. (2012) considered the Singhora package of rocks, identified by Das et al. (1992) as the Singhora Group, to have suffered pervasive regional deformation prior to the emplacement of a ca. 1420 Ma basaltic intrusion (Das et al. 2011), and hence before the deposition of the rocks above, especially those above DU-2 (Table 1). Das et al. (1992, 2001, 2003), Patranabis-Deb and Chaudhuri (2008), and Dhang and Patranabis-Deb (2012) considered the deformation to be local, and along the faults that were likely associated with basin inversion around or after 1000 Ma. The contact between the Singhora Group and the overlying Chandarpur Group (DU-2; Table 1) has been described as a disconformity without specifying if the rocks immediately below the unconformity were eroded (Das et al. 1992, 2001, 2003).

(6)

(7)

(8)

Saha et al. (2012) considered the contact (DU-2; Table 1) as an angular and erosional unconformity. Patranabis Deb and Chaudhuri (2008) and Dhang and PatranabisDeb (2012) considered the packages of rocks defined as the Singhora and the Chandarpur groups by Das et al. (1992, 2003) to be lateral extensions of each other. They could not find any unconformity along the Singhora–Chandarpur contact (DU-2; Table 1). Das et al. (1992) and previous workers (e.g., Murti 1987; Schnitzer 1971) considered the Chandarpur–Raipur boundary to be an unconformity, presumably erosional (DU-3; Table 1), but Patranabis Deb (2004) and Patranabis Deb and Chaudhuri (2008) argued for a gradational contact with no unconformity. According to Das et al. (1992, 2003), *Sst I* (Rehatikhol; Table 1) is the basal sandstone of the Singhora Group and lies below the Singhora–Chandarpur unconformity (DU-2; Table 1). The geographic locations of the group contacts are shown in Figure 2 (simplified after GSI 2005) and using the nomenclature of Das et al. (1992). The map by Dhang and Patranabis-Deb (2012) does not show this unconformity; the geographic extent of the area mapped by Das et al. (2011) and Saha et al. (2012) does not include the location of this unconformity. Das et al. (1992, 2003, 2011) and Saha et al. (2012) considered the sandstone unit *Sst I* (Lohardih; Table 1) above DU-2 to be decidedly separate from the basal *Sst I* (Rehatikhol), but Dhang and Patranabidh-Deb (2012) have emphatically argued that the two sandstone units are the same. In addition, there is a discrepancy about another unconformity near the top of the succession (PU-2 in Table 1; Patranabis-Deb

JSR

RECYCLED QUARTZ AND UNCONFORMITIES

371

FIG. 2.—Geological map of the Singhora area as per GSI, 2004, 2005. The stratigraphic relationships between the Singhora, Chandarpur, and Raipur groups are in dispute.

and Chaudhuri 2008), which Das et al. (1992) apparently did not recognize and Saha et al. (2012) apparently ignored. The stratigraphic controversy concerning this small area, however, has a much larger context. The other two large Proterozoic basins in the Indian peninsula, the Vindhyan and the Cuddapah, have a lower unit of deformed rocks overlain by virtually undeformed rocks. Could the style of the evolution of the Chhattisgarh basin be similar? If so, were the controlling factors similar and even roughly coeval? Could the time ca. 1420 Ma be related to possible suturing of the Eastern Ghats Mobile Belt (EGMB) with the southern cratons of the Indian Peninsula? If so, the suturing event could have triggered deformation of the lithified sediments in the Singhora area, culminating in its inversion, erosion, subsidence, and subsequent deposition of the rest of the Chhattisgarh Supergroup. If not, one is hard pressed to find a driving force for the putative pervasive regional ductile deformation affecting Singhora and adjacent rock units. The ca. 1400 Ma was also the time when Columbia had already broken apart (Rogers and Santosh 2004; Hou et al. 2008; Vijaya Kumar et al. 2011; Meert 2012; Dasgupta et al. 2013) and small continental masses, including the Ur-cratons in India, were readjusting to form what would be Rodinia in another 400 Ma or so (cf. Bhowmik and Dasgupta 2012; Bhowmik et al. 2012). In short, the issue of whether or not there is an erosional unconformity above the rock units in the Singhora area—the Singhora Group of Das et al. (1992, 2003) – casts a much larger penumbra than its geographic extent. PETROLOGIC DATA

Optical Petrography We examined quartz overgrowths in the major sandstones (numbered I, II, II, and IV; see Table 1) of the succession, using between one and five polished thin sections of each sandstone. We followed a combination of the ribbon and the line method (Galehouse 1971, p. 390–392) for estimating the proportions of quartz grains with multiple and rounded (abraded) overgrowths. In the line method, all grains along an E–W cross-hair in the field of view are counted as the stage is moved E–W,

stepped up and moved W–E. In the ribbon method, the line is replaced by a N–S band, called a ribbon, the width of which is larger than the largest grain. The counts so obtained do not have the quality of proportionality between point percentage and volume percentage of a grain type but is more representative of grains with low and even trace abundance. The number of grains so examined ranged from about 500 to 2000 per sandstone. *Sst I* (Lohardih/Rehatikhol) occurs immediately above the granitic basement, i.e., above the unconformity DU-1 and PU-1 (Table 1). The sandstone is coarse-grained, locally pebbly, and arkosic with many granitic rock fragments. The pebbles are mostly granitic; a few are metaquartzites of diverse composition (e.g., hematite or jasper quartzite). Less than 10% of the quartz grains have overgrowths (Fig. 3A). We have not found quartz grains with multiple overgrowths. Wani and Mondal (2010) mention ‘‘multicycle quartz grains floating in calcite cement,’’ which could be silica overgrowths partially replaced by a subsequent generation of calcite cement. All indications are that *Sst I* is petrologically a first-cycle arenite, confirming the erosional nature of the nonconformity below. This confirms the obvious, but nonetheless needs to be noted in view of what follows. *Sst II* (Kansapathar) occurs above the disputed unconformity DU-2 (Table 1). The sandstone is a fine- to coarse-grained quartz arenite with many glauconite grains (Fig. 3B). More than 90% of the detrital quartz grains have overgrowths. The grains do not have obvious multiple overgrowths (Datta 2005). In our extensive systematic search, we found only two quartz grains with suspected abraded or rounded overgrowths (Fig. 3C). Petrologically, therefore, *Sst II* with rare recycled quartz grains from older quartz-cemented sandstone is very likely a first-cycle quartz arenite, as Datta (2005) had inferred. However, Khan and Mukherjee (1990) reported second-cycle quartz grains from a lithostratigraphically equivalent formation from the southwestern part of the Chhattisgarh basin. They assigned *Sst I* as the provenance of these grains. *Sst III* (Deodongar) occurs as a mappable member of a thick carbonate unit (the Chandi Formation) in the western part of the basin above the unconformity DU-3 (Table 1). The sandstone is a fine- to

372

JSR

A. BASU ET AL.

FIG. 3.—Optical images of the four sandstones under consideration with cross-polarized light. A) *Sst I* (TS# 20/07): four rounded detrital quartz grains with overgrowths. B) *Sst II* TS# 108/07): all quartz grains have overgrowths; the concave boundaries of two adjacent quartz grains (see thick hollow arrows) are reminiscent of eolian shapes (cf. Werner and Merino 1997); glauconite grains (GL) are at lower right and left-middle. C) *Sst II* (TS# 108/07): OVG-1 is possibly a rounded overgrowth indicating a recycled origin of the quartz grain; OVG-2 is the last cement. D) *Sst III* (TS# 741109): the quartz grain at the center shows abraded and rounded overgrowths (OVG-1); OVG-2 is the last cement. E) *Sst III* (TS# 741109): quartz grain with two generations of abraded and rounded overgrowths (OVG-1 and OVG-2); OVG-3 is the last cement. F) *Sst IV* (TS# IS-1): one large altered tuffaceous rock fragment (RF-1) in the center and another to its right (RF-2) derived from the 1000 Ma tuff–volcaniclastic mudstone immediately below.

medium-grained quartz-cemented quartz arenite consisting mostly of monocrystalline detrital grains. About 4% of the detrital grains have double and even triple overgrowths. Some of the first-generation overgrowths show abrading or rounding above which a second generation of overgrowth can be seen (Fig. 3D, E). A few recycled quartz grains are bean-shaped and may have had an eolian origin (Werner and Merino 1997). The most likely source would be the eolian beds in *Sst I* (e.g., Chakraborty and Paul 2005; Patranabis Deb and Chaudhuri 2007). There is a possibility that these grains may have been recycled from a 200-m-thick metaquartzite that retain the original texture of its quartz-arenite-protolith and belong to the Archean eolian Karutola Formation to the southwest of the basin and beyond the occurrence of the basin-adjacent Dongargarh Granite (Chakraborty and SenSarma 2008). If so, some of the recycled detrital quartz grains would have been extrabasinal. However, the absence of any metamorphiclast in *Sst III* argues against this possibility. Das and Rao (1997) have reported multi-cycle quartz grains from this unit. Petrologically, therefore, *Sst III* is a multi-cycle quartz arenite with a provenance that included quartz-cemented sandstones. *Sst IV* (Sarnadih) occurs above the recently identified unconformity PU-2 (Table 1; Patranabis-Deb and Chaudhuri 2008). The sandstone is a poorly sorted fine- to coarse-grained clay-cemented lithic arenite. Clastic grains are mostly quartz and volcaniclastic fragments; most of the latter have been diagenetically altered to pseudomatrix, although a few partly altered tuffaceous grains survive (Fig. 3F). The fragments resemble the tuffaceous unit immediately below the unconformity (Basu et al. 2008). The quartz grains are angular to subangular and do not show overgrowths. *Sst IV*, therefore, is a lithic arenite reworked mostly from the tuffaceous and volcaniclastic material below.

SEM Color CL We selected a few random locations in one thin section (761109) of *Sst III* for more detailed observation of abraded overgrowths. We used a scanning electron microscope (SEM, FEI Quanta FEG 400) equipped with a GATAN Chroma CL detector to image cathodoluminescence (CL) colors in real time. CL images were acquired simultaneously with standard secondary-electron (SE) and backscattered-electron (BSE) images. Our observations not only confirmed the abraded nature of the recycled overgrowths that were identified optically, but also provided new insights into the emplacement of the different generations of quartz overgrowths. Correspondence of abraded overgrowths between optical (crosspolarized light; 1-l red-plate inserted) and SEM-color-CL images can be seen in Figure 4A and B, respectively. Details of the products of the cementation process, however, are more clearly seen in CL images (Fig. 5A, B, C). For example, what appears to be a simple abraded overgrowth at optical scale (Fig. 5A) can be seen to be uneven in detail (Fig. 5B). The overgrowth is more complex, and what appears to be one abraded overgrowth under the optical microscope (Fig. 5A) actually carries yet another overgrowth that has not been abraded (Fig. 5B, C). This CL image (Fig. 5C) shows that the overgrowth on the lower grain has a different color than those on the other grains in the field of view, suggesting that this overgrowth precipitated from a pore fluid apparently with different trace-element concentrations. The optical image (Fig. 5A) does not give a clue to the difference. The CL images (Fig. 5B, C) also show how the late pore-filling cement fractured and invaded the overgrowths on a recycled quartz grain, possibly through its own force of crystallization (cf. Maliva and Siever 1988).

JSR

RECYCLED QUARTZ AND UNCONFORMITIES

373

FIG. 4.—A) Optical image of *Sst III* (TS# 761109; location 5) in cross-polarized light with the red-plate ‘‘in,’’ which shows the optical continuity of original grains into the overgrowths. Overgrowth I (OVG-1) is clearly abraded, and the detrital boundary of the recycled grain is marked by a line of dust. Later overgrowths (OVG-2) include all post-abrasion precipitations including in situ pore-filling cement; B) SEM Color CL image of the grain in the center of Part A in which the abraded overgrowth and the cement are marked by different CL colors.

In addition, the CL observations show at least three different color groupings of the original grains, and suggest that at least three different source rocks supplied quartz grains to *Sst III*. However, any detailed investigation of the disposition of quartz overgrowths and cementation, as well as that of provenance is beyond the scope of this paper, which examines abraded and inherited overgrowths vis a` vis unconformities. DETRITAL-ZIRCON GEOCHRONOLOGIC DATA

Previously published U-Pb SHRIMP ages of detrital zircons from *Sst I, *Sst II*, and *Sst IV* (Bickford et al. 2011a) supplement the assessment of presumed unconformities under consideration. The spectrum of the ages of detrital zircons in (a) *Sst I* (the basal Lohardih/Rehatikhol formations) is unimodal (Fig. 6A) with one major peak at ca. 2470 Ma, reflecting supply from the granitic basement; (b) *Sst II* is also unimodal (Fig. 6B) with one major peak at 2485 Ma

reflecting supply from the granitic basement; and (c) *Sst IV* is polymodal (Fig. 6C), with major peaks at ca. 1000 Ma, ca. 1230 Ma, ca. 1570 Ma, and ca. 2500 Ma, indicating supply from multiple sources; the major gaps are between 1400–1500 Ma and 1900–2200 Ma. The Singhora tuff that erupted between 1500 and 1400 Ma very likely did not contribute any detrital zircon to *Sst IV*. There are no geochronologic data on detrital minerals in *Sst III*. All data are summarized in Table 2. DISCUSSION

Stratigraphic Inferences Outcrops of *Sst I* skirt the southern margin of the Chhattisgarh basin. The sediments in *Sst I were derived from the Archean granite–greenstone– older-metamorphic basement and not from any silica-cemented unmetamorphosed quartz arenite. The field occurrence, presence of granitic rock

FIG. 5.—A) Optical image in cross-polarized light with the 1-l red-plate ‘‘in,’’ showing optical continuity of original grains into the overgrowths; no separate pore-filling cement or details within the overgrowths are discernible. *Sst III* (TS# 761109; location 3). B) SEM-CL color image of approximately the same field of view as in Part A. Much greater detail of cement paragenesis is seen. See Part C for more detail. *Sst III* (TS# 761109; location 3). C) Highermagnification and higher-resolution image of a segment of the areas shown in Parts A and B, with raw CL colors showing the paragenesis of overgrowths. The abraded overgrowth 1 is followed by overgrowth 2 (note sharp crystal edges); in situ pore-filling cementation (reddish) follows fracturing and not only intruding into the overgrowths but also into the edge of the original detrital quartz grain. *Sst III* (TS# 761109; location 3).

374

JSR

A. BASU ET AL.

FIG. 6.—Age spectra of detrital zircons recalculated from data in Bickford et al. (2011a) using Isoplot 3.71 (Ludwig 2008). A) An essentially unimodal age distribution with a peak at 2470 Ma for *Sst I*; B) an essentially unimodal age distribution with a peak at 2485 Ma for *Sst II*; C) a polymodal age distribution with important peaks at 1006, 1230, 1570, and 2500 Ma for *Sst IV*; note the gap between 1400 Ma and 1500 Ma.

fragments, and unimodal character of detrital-zircon ages (Fig. 6A; Bickford et al. 2011a; their figs. 7, 8, 9) clearly indicate the erosional nature of unconformities DU-1 and PU-1 (Tables 1, 2). The near absence of recycled quartz grains in *Sst II* suggests that quartz-cemented grains in *Sst I* very likely did not supply detritus to *Sst II*. The absence of detrital zircons of ages younger than ca. 2470 Ma in *Sst II* (Fig. 6B; Bickford et al. 2011a; their figs. 10, 11) above the putative erosional unconformity DU-2 indicates that the ‘‘unconformity’’ may not have reached the ca. 1400–1500 Ma tuff below (ca. 100 m) and did not rework any zircon from that tuff (Chakraborti 1997; Das et al. 2009). This evidence from detrital quartz and zircon indicates that the sedimentary units below *Sst II* remained largely buried during the deposition of *Sst II* while detrital clastics from the ca. 2500 Ma basement bypassed the rock units below DU-2. The above does not entirely rule out the possible existence of an unconformity (DU-2) because (a) a disconformity surface (as shown by Das et al. 1992, 2001, 2003) does not have to be erosional, and (b) the eroded detritus, if any, might have been deposited elsewhere. The presence of multicycle detrital quartz grains (Figs. 3D, E, 4A, B, 5A, B, C) in *Sst III* (see also Das et al. 1992, p. 279) indicates that an

erosional unconformity (DU-3) cut into older quartz-cemented arenites. Tidal bars make up *Sst III* (Mukherjee and Khan 1996), indicating deposition at the shoreline. It is possible that either *Sst II* or *Sst I* or both were exposed near the margin of the basin and supplied detritus to *Sst III*. Given the geology of the area, *Sst II* and *Sst I* are the best candidates to be the sources for the recycled quartz grains. They occur up to 900 m and 1200 m respectively below *Sst III* according to thickness estimates given in Das et al. (1992, 2003). The postulated basin-margin uplift would be significant. Note, however, that the correlations of stratigraphic units across the Chhattisgarh basin by Das et al. (1992) are in error by at least 800 m (Bickford et al. 2011b). Unfortunately, no other comprehensive stratigraphic column has been proposed. Whether or not zircon grains were recycled from *Sst I* and *Sst II* into *Sst III* cannot be determined because of the lack of ages of detrital zircon or monazite from *Sst III*. There is a possibility that the Dallirajhara or the Dongargarh Group of rocks including the Karutola metaquartzite in the Kotri–Dongargarh orogen to the southwest of the basin were uplifted and supplied detritus to *Sst III*. It is not clear why mostly monocrystalline quartz grains would survive in such detritus and that there would be an absence of polycrystalline quartz grains consisting of subgrains with sutured contacts

TABLE 2.— Summary of petrologic and geochronologic data pertaining to *Sst I, II, III, and IV*. Formation Gondwana Unconformity *Sst IV* Unconformity *Sst III* Unconformity *Sst II* Unconformity *Sst I* Unconformity Granitic basement n/a 5 not applicable

Recycled Quartz Grains n/a no yes yes extremely rare no no n/a

Other Rock Fragments

Principal Ages of Detrital Zircon

n/a yes yes; tuffaceous mudstone yes no

n/a

no

ca. 2500 Ma No ca. 2500 Ma yes n/a

yes; granitic yes n/a

ca. 1000, 1230, 1570, and 2500 Ma yes not available

JSR

RECYCLED QUARTZ AND UNCONFORMITIES

(cf. Young 1976). There is also no evidence of any characteristic grain from either of these metamorphosed units in *Sst III* or elsewhere. Therefore, we consider this possibility unlikely. The presence of (a) ca. 1000 Ma and ca. 2500 Ma detrital zircons (Bickford et al. 2011a; their figs. 12, 13, 14), and (b) tuffaceous clasts in *Sst IV* (Fig. 3F; Basu et al. 2008; their figs. 3b, 4) above the presumed unconformity PU-2 indicate that the unconformity cut into the 1000 Ma tuff (Table 1) that occurs immediately below this arenite. This unconformity was not recognized by Das et al. (1992). It appears that because of the paucity of outcrops, identical-looking buff shale and carbonate rocks, and poorly constrained K-Ar ages of glauconite (Kreuzer et al. 1977), *Sst III* and *Sst IV* were miscorrelated, as discussed by Mukherjee and Das (1993), and Mukherjee and Ray (2008, 2010). The problem was compounded through printing errors by the Geological Survey of India (GSI 2005). Thus, the stratigraphic column of Das et al. (1992) that lacks this unconformity has persisted in the literature, including recent monographs (e.g., Naqvi 2005; Ramakrishnan and Vaidyanadhan 2008). In general, erosional unconformities and major unroofing are consequences of tectonic uplifts or sea-level falls in the extrabasinal or intrabasinal source areas of sedimentary successions. In the event that we find recycled sedimentary rock fragments or quartz grains from within a basin (as in the Chhattisgarh basin), a tectonic event should have uplifted the lower units of a succession to make them liable to erosion. Such observations have far-reaching implications for the tectonic evolution of a basin. But, a thorough discussion of the issue is beyond the scope of this paper. With respect to the Chhattisgarh basin, the proposed unconformity PU-2 was probably caused by basin inversion and consequent uplift of the Chhattisgarh succession (Patranabis-Deb and Chaudhuri 2010). However, the tectonic event causing the putative angular unconformity DU-2 (Saha et al. 2012), for which there is no obvious evidence of recycled clastics, is elusive at the present state of knowledge. The discussion above shows that the extents of unconformities based on independent datasets such as field observation and detrital-zircon geochronology are compatible with inferences based on the presence or absence of recycled quartz. Thus, our argument about the validity of recycled quartz as an indicator of ‘‘intrabasinal unconformities’’ in the Chhattisgarh basin is sustained. We propose that this argument can be extended to basins that did not have any extrabasinal sedimentary source rocks. Application Unconformities are fundamental to stratigraphy and essential for sequence boundaries. Using recycled quartz grains as a criterion to identify erosional unconformities adds to the tools for exploration, such as for petroleum, uranium, and water. We expect that the presence of recycled quartz grains in drill cores and cuttings will indicate unconformities or extrabasinal unroofing events that might not have been recognized before. This petrographic tool would also be useful in fieldscale exploration where outcrops are not abundant. CONCLUSIONS

(1)

(2)

Recycled Quartz. This study proposes a new tool in applied sedimentary geology. Detrital quartz grains with inherited overgrowths are sedimentary rock fragments that may be of extrabasinal or intra-basinal origin. Their presence in sedimentary rocks identifies intrabasinal erosional unconformities or extrabasinal unroofing. SEM color cathodoluminescence images show nano-scale details and compositional variations within and between different generations of overgrowths.

(3)

375

Status of the unconformity-bounded Singhora and Kharsyia groups (Table 1). This work suggests (a) that the unconformity DU-2, identified as a disconformity is not erosional, (b) that the unconformity DU-3 is erosional, and (c) that the unconformity PU-2 is erosional. However, this work does not resolve any question on the stratigraphic status of either the Singhora Group or the Kharsyia Group. ACKNOWLEDGMENTS

This research has been supported by Indiana University and grants from DST Government of India, and Indian Statistical Institute. An NSF equipment grant to Juergen Schieber (EAR-0318769) provided funds for the purchase of the analytical SEM that was used for acquiring the images used in this report. Purbasha Bhattacharya collected some of the samples used in this study. We are grateful to M.E. Bickford and Purbasha Bhattacharya for their insightful advice on detrital-zircon geochronology and the source rocks of Chhattisgarh sediments, respectively. Tapan Chakraborty, Salvatore Critelli, Eduardo Garzanti, Dhrubajyoti Mukhopadhyay, and Gian Zuffa freely shared their observations and experience. Ruth Droppo prepared the high resolution figures. We greatly appreciate the formal feedback from an anonymous reviewer, Ray Ingersoll, Gerilyn Soreghan, and John Southard, which assisted in correcting oversights and softening our original stance.

REFERENCES

ADAMS, A.E., MACKENZIE, W.S., AND GUILFORD, C., 1984, Atlas of Sedimentary Rocks under the Microscope; Essex, U.K., Halstead, p. 104. AMOROSI, A., AND ZUFFA, G.G., 2011, Sand composition changes across key boundaries of siliciclastic and hybrid depositional sequences: Sedimentary Geology, v. 236, p. 153–163. BALL, V., 1877, On the geology of the Mahanadi Basin and its vicinity: Record of the Geological Survey of India, v. 10, p. 167–186. BASU, A., PATRANABIS-DEB, S., SCHIEBER, J., AND DHANG, P.C., 2008, Stratigraphic position of the 1000 Ma Sukhda Tuff (Chhattisgarh Supergroup, India) and the 500 Ma question: Precambrian Research, v. 167, p. 383–388. BHOWMIK, S.K., AND DASGUPTA, S., 2012, Tectonothermal evolution of the Banded Gneissic Complex in central Rajasthan, NW India: present status and correlation: Journal of Asian Earth Sciences, v. 49, p. 339–348. BHOWMIK, S.K., WILDE, S.A., BHANDARI, A., PAL, T., AND PANT, N.C., 2012, Growth of the Greater Indian Landmass and its assembly in Rodinia: geochronological evidence from the Central Indian Tectonic Zone: Gondwana Research, v. 22, p. 54–72. BICKFORD, M.E., BASU A., PATRANABIS-DEB, S., DHANG, P.C., AND SCHIEBER, J., 2011a, Depositional history of the Chhattisgarh basin, central India: constraints from new SHRIMP zircon ages: Journal of Geology, v. 119, p. 33–50. BICKFORD, M.E., BASU, A., MUKHERJEE, A., HIETPAS, J., SCHIEBER, J., PATRANABIS-DEB, S., RAY, R.K., GUHEY, R., BHATTACHARYA, P., AND DHANG, P.C., 2011b, New U-Pb SHRIMP zircon ages of the Dhamda Tuff in the Mesoproterozoic Chhattisgarh Basin, Peninsular India: stratigraphic implications and significance of a 1-Ga thermalmagmatic event: Journal of Geology, v. 119, p. 535–548. BLATT, H., 1967, Original characteristics of clastic quartz grains: Journal of Sedimentary Petrology, v. 37, p. 401–424. BLATT, H., AND JONES, R.L., 1975, Proportions of exposed igneous, metamorphic, and sedimentary rocks: Geological Society of America, Bulletin, v. 86, p. 1085–1088. CHAKRABORTI, S., 1997, Elucidation of the sedimentary history of the Singhora Group of rocks, Chhattisgarh Supergroup, M.P.: Geological Survey of India, Record, v. 130, p. 180–187. CHAKRABORTY, P.P., AND PAUL, S., 2005, Proterozoic braid-delta deposits: Lohardih Formation, Chattisgarh Supergroup, India: Indian Journal of Geology, v. 78, p. 233– 253. CHAKRABORTY, T., AND SENSARMA, S., 2008, Shallow marine and coastal eolian quartz arenites in the Neoarchean–Palaeoproterozoic Karutola Formation, Dongargarh Volcano-sedimentary succession, central India: Precambrian Research, v. 162, p. 284– 301. CRITELLI, S., ARRIBAS, J., LE PERA, E., TORTOSA, A., MARSAGLIA, K.M., AND LATTER, K.K., 2003, The recycled orogenic sand provenance from an uplifted thrust belt, Betic Cordillera, Southern Spain: Journal of Sedimentary Research, v. 73, p. 72–81. DAS, D.P., KUNDU, A., DAS, N., DUTTA, D.R., KUMARAN, K., RAMAMURTHY, S., THANAVELU, S., AND RAJAYIA, V., 1992, Lithostratigraphy and sedimentation of Chattisgarh basin: Indian Minerals, v. 46, p. 271–288. DAS, D.P., DUTTA, N.K., DUTTA, D.R., THANAVELLU, C., AND BABU RAO, K., 2003, Singhora Group: the oldest Proterozoic lithopackage of eastern Bastar Craton and its significance: Indian Minerals, v. 57, p. 127–138.

376

A. BASU ET AL.

DAS, K., YOKOYAMA, K., CHAKRABORTY, P.P., AND SARKAR, A., 2009, Basal tuffs and contemporaneity of the Chattisgarh and Khariar basins based on new dates and geochemistry: Journal of Geology, v. 117, p. 88–102. DAS, N., DUTTA, D.R., AND DAS, D.P., 2001, Proterozoic cover sediments of southeastern Chhattisgarh State and adjoining parts of Orissa: Geological Survey of India, Special Publication 55, p. 237–262. DAS, P., DAS, K., CHAKRABORTY, P.P., AND BALAKRISHNAN, S., 2011, 1420 Ma diabasic intrusives from the Mesoproterozoic Singhora Group, Chhattisgarh Supergroup, India: implications towards non-plume intrusive activity: Journal of Earth System Science, v. 120, p. 223–236. DAS, S.S., AND RAO, C.N., 1997, Petrography of Raipur Sandstone, Chattisgarh Basin: Indian Association of Sedimentologists, Journal, v. 16, p. 117–125. DASGUPTA, S., BOSE, S., AND DAS, K., 2013, Tectonic evolution of the Eastern Ghats Belt, India: Precambrian Research, in press. DATTA, B., 2005, Provenance, tectonics and palaeoclimate of Proterozoic Chandarpur sandstones, Chattisgarh basin: a petrographic view: Journal of Earth System Science, v. 114, p. 227–245. DHANG, P.C., AND PATRANABIS-DEB, S., 2012, Lithostratigraphy of the basal part of the Chattisgarh Supergroup around Singhora–Saraipali area and its tectonic implication, in Tiwari, R.P., [ed.] Cenozoic Tectonics, Seismology, and Palaeobiology of the Eastern Himalayas and Indo-Myanmar Range: Geological Society of India, Memoir 77, p. 493–515. DOTT, R.H., 2003, The importance of eolian abrasion in supermature quartz sandstones and the paradox of weathering on vegetation-free landscapes: Journal of Geology, v. 111, p. 387–405. DUTT, N.V.B.S., 1964, A suggested succession of the Purana formations of the southern parts of Chhattisgarh, Madhya Pradesh: Records of the Geological Survey of India, v. 93, p. 143–148. ERIKSSON, K.A., CAMPBELL, I.H., PALIN, J.M., ALLEN, C.M., AND BOCK, B., 2004, Evidence for multiple recycling in Neoproterozoic through Pennsylvanian sedimentary rocks of the central Appalachian Basin: Journal of Geology, v. 112, p. 261–276. GALEHOUSE, J.S., 1971, Point Counting (Ch. 6), in Carver, R.E., ed., Procedures in Sedimentary Petrology: New York, Wiley, p. 635. GARZANTI, E., ANDO, S., VEZZOLI, G., AND DELL’ERA, D., 2003, From rifted margins to foreland basins: investigating provenance and sediment dispersal across Desert Arabia (Oman, U.A.E.): Journal of Sedimentary Research, v. 73, p. 572–588. GSI, 2004, District Resources Map, Mahasamund District, Chhattisgarh (1:25,000), Kolkata, Geological Survey of India. GSI, 2005, Geological and Mineral Map of Chhattisgarh Hyderabad: Geological Survey of India, (1:1,000,000). HOU, G., SANTOSH, M., QIAN, X., LISTER, G.S., AND LI, J., 2008, Configuration of the Late Paleoproterozoic supercontinent Columbia: insights from radiating mafic dyke swarms: Gondwana Research, v. 14, p. 395–409. INGERSOLL, R.V., 1983, Petrofacies and provenance of Late Mesozoic forearc basin, northern and central California: American Association of Petroleum Geologists, Bulletin, v. 67, p. 1125–1142. JOHNSSON, M.J., STALLARD, R.F., AND MEADE, R.H., 1988, First-cycle quartz arenites in the Orinoco River Basin, Venezuela and Colombia: Journal of Geology, v. 96, p. 263– 277. KHAN, M.W.Y., AND MUKHERJEE, A., 1990, Sedimentology and petrography of Chandarpur sandstones, Chhattisgarh Supergroup, around Lohara, Durg District, M.P.: Indian Journal of Earth Sciences, v. 17, p. 44–50. KING, W., 1885, Sketch of the progress of geological work in the Chhattisgarh division of the Central Provinces: Record of the Geological Survey of India, v. 18, p. 169–200. KREUZER, H., HARRE, W., KU¨ RSTEN, M., SCHNITZER, W.A., MURTI, K.S., AND SRIVASTAVA, N.K., 1977, K-Ar dates of two glauconites from the Chandarpur Series (Chhattisgarh /India): On the stratigraphic status of the Late Precambrian basins in central India: Geologisches Jahrbuch, Reihe B: Regionale Geologie Ausland, v. B28, p. 25–36. KUENEN, P.H., 1960, Experimental Abrasion 4: Eolian Action: The Journal of Geology, v. 68, p. 427–449. LUDWIG, K.R., 2008, Isoplot 3.6: a geochronological toolkit for Microsoft Excel: Berkeley Geochronological Center, Special Publication 4, 1–77. MALIVA, R.G., AND SIEVER, R., 1988, Diagenetic replacement controlled by force of crystallization, Geology, v. 16, p. 688–691. MEERT, J.G., 2012, What’s in a name? The Columbia (Paleopangaea/Nuna) supercontinent: Gondwana Research, v. 21, p. 987–993. MUKHERJEE, A., AND DAS, D.P., 2003, Discussion-Reply: Lithostratigraphy and sedimentation of Chhattisgarh Basin: Indian Minerals, v. 47, p. 351–352.

JSR

MUKHERJEE, A., AND KHAN, M.W.Y., 1996, Detailed facies analysis of Deodongar Member, Chhattisgarh Supergroup, Durg–Raipur Districts, M.P.: Indian Journal of Earth Sciences, v. 23, p. 139–146. MUKHERJEE, A., AND RAY, R.K., 2008, SHRIMP ages of zircon in the uppermost tuff in Chattisgarh Basin in central India require 500-Ma adjustment in Indian Proterozoic Stratigraphy: A Discussion: Journal of Geology, v. 116, p. 537–539. MUKHERJEE, A., AND RAY, R.K., 2010, An alternate view on the stratigraphic position of the 1Ga Sukhda tuff vis-a`-vis chronostratigraphy of the Precambrian of the Central Indian Craton: Journal of Geology, v. 118, p. 325–332. MURTI, K.S., 1987, Stratigraphy and sedimentation in Chhattisgarh Basin, in Radhakrishna, B.P., ed., Purana Basins of Peninsular India: Geological Society of India, Memoir 6, p. 239–260. NAQVI, S.M., 2005, Geology and the Evolution of the Indian Plate: New Delhi, Capital, 450 p. PATRANABIS-DEB, S., 2004, Lithostratigraphy of the Neoproterozoic Chattisgarh Sequence, its bearing on tectonics and palaeogeography: Gondwana Research, v. 7, p. 323–337. PATRANABIS-DEB, S., AND CHAUDHURI, A.K., 2007, A retreating fan-delta system in the Neoproterozoic Chattisgarh rift basin, central India: major controls on its evolution: American Association of Petroleum Geologists, Bulletin, v. 91, p. 785–808. PATRANABIS-DEB, S., AND CHAUDHURI, A.K., 2008, Sequence evolution in the eastern Chhattisgarh Basin: constraints on correlation and stratigraphic analysis: The Palaeobotanist, v. 57, p. 15–32. PATRANABIS-DEB, S., AND CHAUDHURI, A.K., 2010, Stratigraphic and tectonic evolution of the Mesoproterozoic Chattisgarh basin in central India: Indian Journal of Geology, v. 80 (2008; pub. 2010), p. 139–155. PATRANABIS-DEB, S., BICKFORD, M.E., HILL, B., CHAUDHURI, A.K., AND BASU, A., 2007, SHRIMP ages of zircon in the uppermost tuff in Chattisgarh basin in central India require 500 Ma adjustment in Indian Proterozoic stratigraphy: Journal of Geology, v. 115, p. 407–415. RAMAKRISHNAN, M., AND VAIDYANADHAN, R., 2008, Geology of India, v. 1: Geological Society of India, 552 p. ROGERS, J.J.W., AND SANTOSH, M., 2004, Continents and Supercontinents: New York, Oxford University Press, 289 p. SAHA, S., DAS, K., CHAKRABORTY, P.P., DAS, P., KARMAKAR, S., AND MAMTANI, M.A., 2013, Tectono-magmatic evolution of the Mesoproterozoic Singhora basin, central India: evidence for compressional tectonics from structural data, AMS study and geochemistry of basic rocks: Precambrian Research, in press. SANDERSON, I.D., 1984, Recognition and significance of inherited quartz overgrowths in quartz arenites: Journal of Sedimentary Petrology, v. 54, p. 473–486. SCHNITZER, W.A., 1971, Das Jungpra¨kambrium Indiens (‘‘Purana System’’): Neugliederung, Stromatolithenfu¨hrung und lithofazielle Vergleiche: Erlanger Geologische Abhandlungen, v. 85, 44 p. SUTTNER, L.J., BASU, A., AND MACk, G.H., 1981, Climate and the origin of quartz arenites: Journal of Sedimentary Petrology, v. 51, p. 1235–1246. TUCKER, M.E., 2001, Sedimentary Petrology, Third Edition: Oxford, U.K., Blackwell, 262 p. VAIDYANADHAN, R., AND RAMAKRISHNAN, M., 2008, Geology of India, v. 2: Geological Society of India, 438 p. VIJAYA KUMAR, K., LEELANANDAM, C., AND ERNST, W.G., 2011, Formation and fragmentation of the Palaeoproterozoic supercontinent Columbia: evidence from the Eastern Ghats Granulite Belt, southeast India: International Geology Review, v. 53, p. 1297–1311. WANI, H., AND MONDAL, M.E.A., 2010, Petrological and geochemical evidence of the Paleoproterozoic and the Meso-Neoproterozoic sedimentary rocks of the Bastar craton, Indian Peninsula: implications on paleoweathering and Proterozoic crustal evolution: Journal of Asian Earth Sciences, v. 38, p. 220–232. WERNER, B.T., AND MERINO, E., 1997, Concave sand grains in eolian environments: evidence, mechanism, and modeling: Journal of Sedimentary Research, v. 67, p. 754– 762. YOUNG, S.W., 1976, Petrographic textures of detrital polycrystalline quartz as an aid to interpreting crystalline source rocks: Journal of Sedimentary Petrology, v. 46, 595– 603. ZUFFA, G.G., 1987, Unravelling hinterland and offshore pelaeogeography from deepwater arenites, in Legget, J.K., and Zuffa, G.G., eds., Marine Clastic Sedimentology: Concepts and Case Studies: London, Graham and Trotman, p. 39–61. Received 28 July 2012; accepted 31 December 2012.