Microbially originated wrinkle structures on sandstone and their - - PowerPoint PPT Presentation

Microbially originated wrinkle structures on sandstone and their stratigraphic context: Palaeoproterozoic Koldaha Shale, central India

Santanu Banerjee and S. Jeevankumar Department of Earth Sciences Indian Institute of Technology Bombay

Evolution of Life on Earth

Precambrian (4500- 540 Ma)

Evolution of life since its formation around 4500 Ma Paleozoic- Skeletal fossils appear Soft bodied metazoans appear before that Mesozoic- age of reptiles Microbes dominate the earlier part of earth’s history First record of microbe preserved in rocks- 3500 Ma Up to 540 Ma microbes were abundant This paper discusses about microbial life on sea floor around 1600 Ma

65 Ma 225 Ma 540 Ma

Stromatolites- earth’s oldest fossils

Vindhyan, central India 1.7 Ga old

Microbially originated structures in limestones Stromatolites are reported from rocks as old as 3500 Ma, so far > 500 reports, note cyclicity in this example

Stromatolites from shark bay world heritage site

From www.aussieactionadventures.com

Sedimentologists learn from modern- apply the concept in ancient rocks Hypersaline conditions favour microbial growth and form stromatolites

Search for microbial mats in modern clastic sediments in hypersaline environments

Coastal areas west of Baroda

Study area- geological map

• Map prepared by Geological Survey of India
• The age of rocks of importance - 1.6 Ga
• Oldest rocks 1.7 Ga, youngest around 0.6 Ga

Field investigations

• Finding proper sections, mostly

we find soil covers

• Vertical sedimentary sections were

studied in mines

• Note variation in rock types
• Document sedimentary features
• Note the general trend in

lithological variation

• Interpretation of depositional

environments

• Interpretation of sea level

fluctuations – rise or fall

• Collect data for graphical

representation of the section

• Features on bedding surfaces cannot

be seen

Search for bedding surface features

• Search for features occurring on the bedding surfaces (red arrow)
• Collect samples for laboratory investigations

Varieties of wrinkle structures- positive relief

Varieties of wrinkle structures – negative relief

Optical microscopic investigations

0.1 mm 0.5 mm

Thin (<1 mm), dark cover at the top surface Wavy, crinkly carbonaceous fragments on magnification Suggest microbial growth soon after formation- do not confirm

Possible microbial filaments under SEM

Wrinkle varieties in vertical sedimentary succession

Rocks becoming coarser upward Sea level is falling Finer wrinkles forming in the deeper part (50-200 m) of the sea Coarser wrinkles forming in the shallow seas (>10-50 m)

Conclusions

• Discusses about microbial mat originated sedimentary

structures in 1.6 Ga-old sandstones

• Most previous reports of microbial structures are from

limestone

• Explains origin of wide range of wrinkle structures formed by

microbe-sediment interaction

• Discusses variation of the wrinkle structures in vertical

sedimentary succession in response to sea level fall Thank you