The neglected Stromatolitic fossils of Goa
What are “stromatolites” ?. These are internally-laminated,
macroscopic sedimentary structures, commonly of biological origin which form
the dominant part of Earth’s early fossil record and so provide a potentially
important source of information about early life. This article reports the new
discovery of Archean (more than 2500 million years old) stromatolites-vast
fossilized algal mats on border of Goa, inside
Mahadayi river basin among metasedimentary rocks. Time is running out for
fossil researchers because the strong currents of Mahadayi have eroded the
stromatolites. These mats are not the routine major or minor folds found in
metasedimentary rocks. There are stromatolitic carbonates as well. A group of
trekkers had visited the source of Mahadayi river. In the bed of the river and
on the banks they came across strange rocks. The students of Geology amongst
them could not identify these strange formations. One of the leader of the
trekkers, Prakash Parienkar who teaches at Konkani department of Goa University
showed me the photographs. One of these is published alongwith this article.
After minute examination of the images and based on geology and
lithostratigraphy of the Mahadayi basin which is full of detrital Archean
metasediments and after consulting other published sources on stromatolitic
deposits in India and outside, I arrived at the conclusion that upper Mahadayi
basin is a vast, unexplored, neglected treasure trove for archean fossil
hunters with stromatolite fossils being relatively abundant. But the currents
are eroding these fossils. It is virtually a Archean paleontological museum.
Only fools would ignore its’ monumental scientific, academic , intellectual
potential. The Mahadayi river basin is part of Western Dharwar Craton
(WDC) of Meso-archaean age. WDC shows greenstone belts, tonalite-trondhjemite
gneisses, high-Mg basalts, komatites with metavolcanics and meta-sedimentary
rocks. Among these mafic and ultramafic volcanic rocks, arenites, phylites,
polymictic and oligomictic conglomerates, greywackes, banded iron formations
and carbonates are dominant. What’s practical importance of discovery of
stromatolite fossils in Goa?. My attention was
drawn to Geologist Hofmann’s 1973 paper on stromatolites from which I take
liberty to quote the relevant part- “ Stromatolites are remarkable structures.
They are so primitive and simple, and yet so complex and versatile. Not so long
ago still regarded by some as curious objects, these laminated biosedimentary
structures are now intensively studied. They provide information whose
importance spans more than 2.9 billion years of time and three of the major
fields of science: geology, biology and astronomy. Within geology, they have
contributed information of use in sedimentology, paleontology, stratigraphy,
paleogeography, geophysics, and even tectonics. They have been called upon: (1)
to identify places of past biotic activity; (2) to interpret ancient
depositional environments and paleosalinities; (3) to make correlations and age
determinations; (4) to place limits of the time of origin of cyanophytes, oxygen-producing
photosynthesis, filamentous habit of algae, and integrated biological
communities; (5) to determine paleocurrent directions; (6) to map ancient
shorelines; (7) to measure past tidal ranges; (8) to time the closest approach
or the capture of our moon; (9) to determine ancient astronomical rhythms such
as the length of the synodic mont h and the number of days per year; (10) to
determine rates of sediment accumulation; (11) to determine paleolatitudes;
(12) to prospect for base metal deposits; (13) to prospect for microfossils;
(14) to make top-determinations in deformed sequences; (15) to serve as tourist
attractions.” A huge amount of work has been done after Hoffman’s well
delivered paper at the 46th Annual Meeting of the Society of Economic Paleontologists
and Mineralogists, April 18, 1972, Denver,
Colorado. Extraordinary claims
need extraordinary evidence. Late Anant Dhume who wrote “cultural history of Goa” as surveyor had toured the whole state in his field
based career of more than half a century. He could read geomorphology of Goa like a book. While editing his book I gained a lot of
new knowledge about paleontology of Goa. He
had mentioned about discovery of a marine conch at Rive, Thana, Sattari at a depth of half metre. I
had asked him in 1984- “how is it possible to get marine fossils in
Sattari, that too at an altitude of few hundred metres and more than 50 kms.
from the sea?”. He attributed it to paleotectonic activity. The detrital
metasediment in Mahadayi basin points to the possibility of violent tectonic
activity before the formation of Deccan flood
basalt and orogeny of western ghats. Before publication of this article people
would not have believed that the upper basin of Mahadayi
river in Sattari was once ( at least 2.5-3 billion years ago) below the sea or
was a shallow paleoseashore/ paleointertidal zone or may be even a paleoreef.
And this sea or seashore was teeming with life as shown by the extraordinarly
rich stromatolitic fossil rocks. Why stromatolite fossils were not previously
reported?. Firstly there is poor emphasis on paleontology in education and
research. The biogenic factors are often forgotten. Then these rocks are not
located in a single place. Most of the time the currents mask or
hide them. The exposed ones get mixed and easily camouflaged within the
surrounding metasedimentary rocks. Not every folded rock is a stromatolite. A
discovery can be made only if focussed, consistent, rational sampling efforts
are done. This is a region of upper Mahadayi basin which no scientist from NIO,
Goa University, GSI or any other educational
or research institutions has ever dared to enter. The terrain and the route is
treacherous. The route is known only to the local people. Visit is possible
only during late winter. According to experts, Stromatolites are shaped by a
complex interaction of physical, chemical, and biological processes, and
identifying unambiguous signatures of life from the preserved morphology of the
structures can be extremely difficult. There is also need to identify the
activities of benthic microbial communities. But such attempts face problems
due to the process of diagenetic alteration, particularly recrystallization: a
diagenetic process that commonly affects the chemical (precipitated) sediments
with which stromatolites are often associated. Goa
is teeming with such macro and microfossils. But palaeontology of Goa is a totally neglected subject.
