| Carbonaceous megaremains from the
Neoproterozoic Owk Shales Formation of the Kurnool Group, Andhra Pradesh, India |
Mukund Sharma and Manoj Shukla
Birbal Sahni Institute of Palaeobotany, 53, University Road, Lucknow 226 007, India
An assemblage of carbonaceous compression and impressions recorded from the Owk Shales Formation (OSF) of the Kurnool Group include, Chuarid (Chuaria circularis), Tawuid (Tawuia sp.), Ellypsophysid, Moranid and Beltinid remains. The presence of Chuaria-Tawuia assemblage provides strong evidence of correlation with the assemblage of Rewa and Bhander groups of Vindhyan Supergroup and to some extent with the Halkal Formation of the Bhima Group. Chuaria-Tawuia assemblage is being considered as a potential biostratigraphic marker. In this communication we report varied groups of carbonaceous compression and impressions from the OSF of the Kurnool Group. On the basis of the present fossil assemblage, the OSF is considered to be of the Neoproterozoic age.
MEGASCOPIC remains are well recorded from the various 1.1–0.7 Ga-old sedimentary successions of the world1,2. In India, carbonaceous megascopic compression and impressions have previously been recorded from the Vindhyan succession of central India3–9 and Bhima Basin10–14 of south India. A sketchy report has also been made from the Kurnool Basin15. We report here a very well preserved and diversified megascopic assemblage from the Owk Shales Formation (OSF) of the Kurnool Group, and discuss its biostratigraphic significance in the light of other discoveries of carbonaceous megaremains.The OSF is part of the Kurnool Group of the Cuddapah Supergroup exposed in the Kurnool District, Andhra Pradesh. It conformably overlies the Narji Limestone Formation and grades upwards into massive Panium Quartzite Formation16–18 (Figure 1). OSF is a well-laminated, thin-bedded unit with shales, siltstones, and silty-clay stones. The shales are noncalcareous; light grey, buff, and khaki in colour. These shales intercalate with thin-bedded silty claystone in the lower part of the OSF and siltstone in the upper part of the OSF. The carbonaceous compression and impressions reported here are found in abundance on bedding planes at the shale-siltstone, and shale-claystone junctions of the OSF.
The first reference of the occurrence of
carbonaceous remains (presently considered biogenic) was made by King16 who
reported, ‘these are somewhat like cycloid shales of fish, but no organic structure
has been recognized in them’. This observation suggests that although King noted
these dark-spotted features on the bedding surface of the shale, he did not consider these
as biogenic in origin. Later, Rajurkar15, in a very brief communication,
restudied these and compared the circular structures with Fermoria – a
genus presently being
considered as a junior synonym of the Chuaria Walcott. He did not however mention
the other forms present in the OSF. We have noted several additional types of carbonaceous
remains. The morphotype diversity of the carbonaceous remains, present in Precambrian
sediments, has been divided into several groups2. On the same lines the present
occurrence has been described as well.
The collection of fossil assemblage described here has been made from the eastern part of the hillock 1445, situated in the village Ankireddipalle (15° 07˘ :78° 03˘ ) of the Kurnool district of Andhra Pradesh, south India (Figure 2). The exposed thickness of OSF at this place is about 15 m. While the lower 5 m part is dominated by claystone beds, the rest of the 10 m of succession is comprised of shale–siltstone alteration (Figure 3). The figures specimens are deposited in the repository of Birbal Sahni Institute of Palaeobotany, and catalogued under the BSIP Museum Numbers (38078–38082, slide number 12055), as cited in the individual figure captions. The present study of the fresh collection shows that the carbonaceous megafossil assemblage recorded from the OSF belongs to the following 5 groups.
Chuarid remains: The specimens are smooth-walled spheroids preserved as carbonaceous discs, ranging in size from 1–5 mm (E = 3.11: N = 261) (Figures 4 c–f ). Occasionally a few forms show short, rounded extension at one place. Ford and Breed19 placed the size range of the Chuaria from 0.5 to 5 mm. Vidal20 from the study of assemblage in Visings Formation extended the lower extent of the size parameters to 0.09–0.2 mm and the upper limit up to 3 mm. Later, Vidal and Ford21 included specimens as small as 70 µm in diameter in the category of Chuaria circularis. This indicates that there is still no unanimity about the lower size limit of Chuaria. However, the upper size limit is generally agreed to be 5 mm.
Figure 1. The
lithostratigraphy of the Kurnool Group36,37.
Figure 2. Locality
map of the fossil-yielding area showing Ankireddipalle village from where the carbonaceous
compressions have been recorded.
Figure 3. Lithostratigraphic succession of the strata exposed at Ankireddipalle village, Kurnool district, Andhra Pradesh, showing Narji Limestone Formation, Owk Shales Formation, and Panium Quartzite Formation, and indicating the levels of abundant occurrence of carbonaceous compression/impressions.
In India, the Chuaria circularis recorded from Suket Shales of the Semri Group, Vindhyan Supergroup, is in the size range of 0.5–1.25 mm (ref. 22) and the same noted from the Rohtas Limestone Formation ranges in size from 2–4 mm (ref. 14). The specimens recorded from Rewa Group are 1.0 to 3.7 mm with the average being 2.0 mm (ref. 8). The Chuaria specimens reported from the Bhander Group range from 0.2 to 5.1 mm, with a mean of 1.54 mm (ref. 9). Thus, the Vindhyan Chuaria specimens range in size from 0.5–5.0 mm. The Bhima specimens are 2–10 mm in diameter with a mean of 3 mm (ref. 14). Thus, the Indian records are in the range of 0.5–10 mm with a mean of ~5 mm.
Chuaria circularis is one of the carbonaceous remains from the Precambrian sediments which has been studied in great detail by several workers. These studies were undertaken to understand the nature and affinity of Chuaria with other groups. Although so far no conclusive proof is available about its affinities, it has been assigned to various groups. Based on the size parameters, Chuaria is categorized under acritarch, comparable to Leiospherids. Sun23 considered this to belong to filamentous algae belonging to Nostoc which forms a ball. On the contrary, Gussow24 considered this as a planktic form, while Steiner25 considered this as micro- to macroscopic (? Chrocoocal organized) colonial prokaryote. But comparison with Nostoc does not hold good because Nostoc has a thin outer covering rather than the thick outer covering found in Chuaria, which may be original or taphonomic. For example, in artificially fossilized prokaryote, an organically impregnated envelope of prokaryotic colonies may simulate large unicells comparable to Chuaria. Thus, it is more likely that Chuaria is more akin to the large-sized prokaryote which has a thick outer coating.
Figure 4. a, Section of Oak
Shales Formation exposed at Ankireddipalle village; b, close up of the
strata yielding abundant carbonaceous megaremains; c–f,
impressions and compressions of Chuarid remains (Chuaria circularis) showing
solitary linear arrangement and ruptured outer covering of Chuaria. c,
Specimen no. BSIP-38078; d and e, Specimen no. BSIP-38079; f,
Specimen no. BSIP-38080; g, Tawuid
remain, specimen no. BSIP-38081; h, Moranid remain, specimen no. BSIP-38082;
i, Ellipsophysiod remain, specimen no. BSIP-38078; and
j, Beltanid remain, BSIP slide no. 12055 England Finder Reading K 2/13.
Tawuid remains: These are short rectilinear, smooth tomaculate forms which are slightly bent or curved and are very few in number. The population density of Tawuia and tauwuid remains is very low in comparison to Chuarid remains in OSF. Their length ranges from 2 to 6 mm, and the width ranges from 1 to 4 mm (Figure 4 g). The specimens recorded from the Rewa Group have a length range between 3.3 mm and 5 mm, and a width range between 1.6 mm and 2.94 mm (ref. 8). In the Rewa Group, specimens show well-developed foldings and creases along the length of the form. Tawuia specimens have also been recorded from the Bhander Group. These specimens have a width of 0.8 to 1.6 mm with mean as 1.14 mm (N = 8), the maximum recorded length is 10.3 mm; cross wall and annulations have not been recorded in these specimens. Sharma and Shukla13 and Maithy and Babu14 recorded Tawuia from the Halkal Formation, Bhima Group, which are smooth, ribbon-like films, short and slender, straight or mostly twisted, parallel-sided with a size range of 15–30 mm in length, and 0.1 to 3 mm in width. It is common knowledge that Chuaria and Tawuia occur together, but the converse is not true. This feature has been noted in the OSF assemblage as well.
Hofmann (in Hofmann and Aitken26) considered the affinities of Tawuia as either algal (Phaeophyte?) or probably metazoan; however, later he opined that both Chuaria and Tawuia are eukaryotic algae1. Duan27 considered Chuaria and Tawuia as advanced multicellular algae in the process of evolution. Sun23 mentioned that Chuaria and Tawuia are closely associated because of their gross confirmation. This has been further supported by statistical data presented by some researchers1,28.
The occurrence of Tawuia in the OSF assemblage is very characteristic of Neoproterozoic successions, and is helpful in establishing the age of OSF.
Ellypsophysid remains: The ellipsoidal and ovate to spatulate forms are included in this category, and considered intermediate between Chuarid and Tawuid remains. The specimens belonging to this category are miniature Tawuia type (Figure 4 h). Such specimens have been described as belonging to the genus: Nephroformia, Pumilibaxa, and possibly Glossophyton26. Specimens range in length from 2–5 mm and in width from 1–3 mm. These forms are perhaps related to longfengshanids type of carbonaceous organic remains attached to stalk, which at some point of time got detached and were therefore preserved2,26. Benthic macroalgae, Longfengshania and Paralongfengshania, occur in 0.9–0.85 Ga-old Qingbaikuouan Changlongshan Formation of the Yenshan Ranges29. This once again stresses the conjecture that Chuaria-Tawuia and ellypsophysid remains are characteristic of Neoproterozoic, the world over.
Moranid remains: These are large ellipsoidal to irregular, rounded impressions that do not show wrinkles, and are considered to lack or have a less-developed physically resistant wall. The microfossils occurring in the OSF range in size from 1–8 mm in length, and 2–4 mm in width (Figure 4 i). These forms appear to have affinity with the genus Morania and have been compared to the modern free-floating globoidal colonies of the cyanobacterium Nostoc30,31.
Beltinid remains: This group includes angulate films of carbonised matter. The characteristic form of this group, Beltina danai, has been noted in the OSF assemblage. These films may represent either diverse remains of no special morphological shape or else just a structureless biogenic matter, or perhaps fragments of prokaryotic mats or colonies, or eukaryotic algae (Figure 4 j). The type material reported from Montana Belt Terrain, Greyson Shale Formation, Deep Creek Canyon, Montana, is fragmentary, angular carbonaceous compression as well as millimetric to centimetric size polygon, devoid of any ornamentation32. The specimens recorded from Halkal Formation of Andola Subgroup are typically similar to the type material13,14. The OSF specimens are small pieces of carbonaceous remains and similar to the type material.
The carbonaceous remains have been recorded from Palaeoproterozoic33, Mesoproterozoic34, and Neoproterozoic1,2. Chuaria has been even recorded from the Cambrian35. In Indian Precambrian basins, Vindhyan successions have been widely studied for both their micro- and megapalaeobiological remains. Chuaria and Tawuia have been recorded from the Semri3,5–7, the Rewa8, and the Bhander Groups9 in central India. These occurrences indicate that carbonaceous remains were occurring widely during the Vindhyan Supergroups over a long period of time spanning from Semri to Bhander groups.
Similar carbonaceous remains are also known from the Bhima Basin11–14. Bhima assemblage is quite diversified in comparison to the other occurrences, but Chuaria and Tawuia are important components of this assemblage. A comparison of Vindhyan assemblage, comprising of Chuaria and Tawuia, with Bhima and Kurnool assemblages shows that these are akin in shape, size, and diversity. Besides, the host lithologies are markedly similar not only at these two areas but also in the OSF of Kurnool Basin. Though lithologically the host sediments are similar in all the three localities, the assemblage recorded from the Bhima Basin shows greater diversification in taxa than those from the OSF and Vindhyan successions. The occurrence of these forms in buff shales or silty shales or siltstone/claystone indicates that their preservation is facies-controlled. This similarity suggests the affinity of this assemblage for shale depositional realm, and points to the planktic nature of the organisms. Smooth forms like Chuaria circularis are considered to be free-floating planktic forms24. It is possible that these were brought in from the open sea and deposited in calm water bodies, such as lagoon/lake, where these shales were deposited and specimens got preserved.
Hofmann1 proposed that the Tawuia-Chuaria assemblage zone can be considered as a potential chrono-bio-stratigraphic marker, and considered the period from 1.1–0.7 Ga as the Chuarian age. Although the OSF assemblage compares closely with the Mesoproterozoic Suket Shales assemblage, which predominantly comprises Chuarid and Grypanid remains, it shows closer similarity with the Rewa and Bhander assemblages which are Neoproterozoic in age. In the present assemblage, neither Grypania spiralis nor any other similar form has been noted. Hence on correlation, we presume that the Kurnool Formation is coeval to the Rewa and Bhander Groups of the Vindhyan Supergroup and the Halkal Formation of the Bhima Group. Thus, the present assemblage indicates a Neoproterozoic age for the OSF. Further search in the area may add to the diversity of palaeobiological remains of the Owk Shales Formation.
ACKNOWLEDGEMENTS. We are thankful to Dr Vibhuti Rai for going through the earlier version of the manuscript. Mukund is grateful to Mr Rajeshwar Tewari, Collector, Kurnool, who helped in our having access to several remote areas of the basin.
Received 31 October 1998; revised accepted 27 January 1999