5:15 PM - 7:15 PM
[BCG06-P07] Characteristics of phosphate stromatolites in the Paleoproterozoic Jhamarkotra Formation, India
Stromatolites are laminated deposits formed by benthic microbial communities. They are mostly composed of carbonate minerals, but rarely composed of phosphate. Well-studied examples of phosphate stromatolites are the Salitre Formation of the Brazilian Neoproterozoic and the Jhamarkotra Formation of the Indian Paleoproterozoic Aravali Supergroup. Neoproterozoic phosphate stromatolites have been found to be associated with Snowball Earth glaciation (Shiraishi et al., 2019), and Aravali Supergroup has also been suggested to be linked to the Huronian glaciation (2.3–2.2 Ga). However, the depositional age of the Aravali Supergroup is less well constrained (2.1–1.7 Ga; Deb and Thorpe, 2004; McKenzie et al., 2013), and it is unclear whether it is related to the Snowball Earth glaciation. The aim of this study is to describe the phosphate stromatolites of the Jhamarkotra Formation and compare them with Neoproterozoic examples to determine how they formed and how they relate to global environmental change. The research methods used were field survey, thin section observation, mineral identification by powder X-ray diffraction and elemental mapping by EPMA.
The study focused mainly on phosphate stromatolites from the Jhamarkotra mine. The layer containing phosphate stromatolites laterally continued over a length of more than 10 km, with a thickness of about 13–37 m. This layer is often folded. In areas where deformation and metamorphism have been prominent, the columnar stromatolites have been either smashed or stretched parallel to the bedding plane, while in some areas deformation and metamorphism have been less prominent. The upper and lower part of the phosphatic layer is mostly dolostone, with sandstone to sandy dolostone in some places. The stromatolites are light grey in the field and consist mainly of fluorapatite, while the surrounding matrix is dark grey in the field and consists mainly of dolomite. The stromatolites sometimes contain spherical structures, possibly representing the traces of oxygen bubbles. The dolomite was often coarse grained, with some quartz veins.
As with the phosphate stromatolites of the Neoproterozoic, the stromatolites were composed mainly of fluorapatite and the matrix was mainly of dolomite. The presence of spherical structures in the stromatolites suggests that oxygenic phototrophs such as cyanobacteria may have been involved in the formation of the phosphate stromatolites, as in the case of the Neoproterozoic example. These results suggest that the phosphate stromatolite characteristics of the Paleoproterozoic Jhamarkotra Formation are similar to those of the Neoproterozoic Salitre Formation, which formed after Snowball Earth glaciation. Future work will be required to constrain the age of formation to clarify the relationship with Snowball Earth glaciation.
References
Deb M., Thorpe R.A. (2004) In: M. Deb and W.D. Goodfellow, eds., Sediment-hosted Lead–Zinc Sulphide Deposits, Narosa Publishing House, pp.246–263.
McKenzie N.R., Hughes N.C., Myrow P.M., Banerjee D.M., Deb M., Planavsky N.J. (2013) Precambrian Research 238, 120–128.
Shiraishi F., Ohnishi S., Hayasaka Y., Hanzawa Y., Takashima C., Okumura T., Kano A. (2019) Sedimentary Geology 380, 65–82.
The study focused mainly on phosphate stromatolites from the Jhamarkotra mine. The layer containing phosphate stromatolites laterally continued over a length of more than 10 km, with a thickness of about 13–37 m. This layer is often folded. In areas where deformation and metamorphism have been prominent, the columnar stromatolites have been either smashed or stretched parallel to the bedding plane, while in some areas deformation and metamorphism have been less prominent. The upper and lower part of the phosphatic layer is mostly dolostone, with sandstone to sandy dolostone in some places. The stromatolites are light grey in the field and consist mainly of fluorapatite, while the surrounding matrix is dark grey in the field and consists mainly of dolomite. The stromatolites sometimes contain spherical structures, possibly representing the traces of oxygen bubbles. The dolomite was often coarse grained, with some quartz veins.
As with the phosphate stromatolites of the Neoproterozoic, the stromatolites were composed mainly of fluorapatite and the matrix was mainly of dolomite. The presence of spherical structures in the stromatolites suggests that oxygenic phototrophs such as cyanobacteria may have been involved in the formation of the phosphate stromatolites, as in the case of the Neoproterozoic example. These results suggest that the phosphate stromatolite characteristics of the Paleoproterozoic Jhamarkotra Formation are similar to those of the Neoproterozoic Salitre Formation, which formed after Snowball Earth glaciation. Future work will be required to constrain the age of formation to clarify the relationship with Snowball Earth glaciation.
References
Deb M., Thorpe R.A. (2004) In: M. Deb and W.D. Goodfellow, eds., Sediment-hosted Lead–Zinc Sulphide Deposits, Narosa Publishing House, pp.246–263.
McKenzie N.R., Hughes N.C., Myrow P.M., Banerjee D.M., Deb M., Planavsky N.J. (2013) Precambrian Research 238, 120–128.
Shiraishi F., Ohnishi S., Hayasaka Y., Hanzawa Y., Takashima C., Okumura T., Kano A. (2019) Sedimentary Geology 380, 65–82.