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Evidence from laminated chert in banded iron formations for deposition by gravitational settling of iron-silicate muds

Rasmussen, Birger and Muhling, Janet R. and Fischer, Woodward W. (2019) Evidence from laminated chert in banded iron formations for deposition by gravitational settling of iron-silicate muds. Geology, 47 (2). pp. 167-170. ISSN 0091-7613. doi:10.1130/g45560.1.

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The deposition of clay-sized particles by settling from suspension within a water column typically forms highly porous (80%–90% water) muds dominated by uniformly distributed and randomly oriented particles. The depositional microfabric of muds is rapidly destroyed during burial, resulting in mudstones with a strong preferred particle orientation. Here we show that laminated chert in banded iron formations from the Hamersley Group, Australia, and Ghaap Group, South Africa, preserve laminae of nanometer-sized particles of greenalite with textures that resemble the fabric of freshly deposited muds—the oldest such fabrics in the rock record. The preservation of the textures implies that the nanoparticles were “frozen” by silica cement on or just below the seafloor, an interpretation supported by their presence in intraformational chert clasts and nodules with strong differential compaction. The co-occurrence of stacks of plates forming thicker aggregates with multiple face-to-edge contacts indicates that the greenalite may have been deposited as loosely formed flocs. The common occurrence, in certain horizons, of iron-silicate microgranules dispersed in the greenalite mud resembles aggregates of clay particles documented in modern muds. These textures support models for active seawater precipitation of the iron-rich phases and point to collision and flocculation of particles during sediment transport. Upon deposition, amorphous silica rapidly occluded the available porosity and entombed the precipitates in a chemically inert cement that limited burial-related compaction and hindered diagenetic reactions, providing a taphonomic view of the earliest mineralogy and texture of iron formations.

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Rasmussen, Birger0000-0001-5052-6797
Fischer, Woodward W.0000-0002-8836-3054
Additional Information:© 2019 Geological Society of America. Manuscript received 5 September 2018; Revised manuscript received 1 December 2018; Manuscript accepted 9 December 2018. Rasmussen received support from the Australian Research Council (DP140100512) and State Key Laboratory of Geological Processes and Mineral Resources Open Research Grant GPMR201802. Fischer acknowledges support of the Simons Foundation Collaboration on the Origins of Life. Support for the acquisition of the scientific drill cores was provided by the Agouron Institute and the NASA Astrobiology Program. SEM, FIB, and TEM analyses were performed at the Centre for Microscopy, Characterization and Analysis at the University of Western Australia, a node of the Australian Microscopy and Microanalysis Research Facility funded from university and government sources. We thank B. Simonson, N. Beukes, and J. Schieber for reviews that helped to improve the manuscript.
Funding AgencyGrant Number
Australian Research CouncilDP140100512
State Key Laboratory of Geological Processes and Mineral ResourcesGPMR201802
Simons FoundationUNSPECIFIED
Agouron InstituteUNSPECIFIED
Issue or Number:2
Record Number:CaltechAUTHORS:20190109-083018705
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Official Citation:Birger Rasmussen, Janet R. Muhling, Woodward W. Fischer; Evidence from laminated chert in banded iron formations for deposition by gravitational settling of iron-silicate muds. Geology ; 47 (2): 167–170. doi:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:92152
Deposited By: Tony Diaz
Deposited On:09 Jan 2019 17:32
Last Modified:16 Nov 2021 03:47

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