Australia: The Land Where Time Began

A biography of the Australian continent 

Banded Iron Formation - Hydrothermal and Resedimented Origins of Precursor Sediments 

The Brockman Supersequence, dating to the Early Palaeoproterozoic, is comprised of banded iron formation (BIF), bedded chert, limestone, mudrock, sandstone, tuffaceous mudstone, breccia, ashfall tuff, as well as basalt and rhyolite in sections that are not reported in this article. Sandstones, limestones, mudrocks and tuffaceous mudstones preserve rhythms of density flows, and as similar rhythm relics have been found in BIF it is implied that density currents were also responsible for the deposition of its precursor sediments. Siliciclastic or mixed siliciclastic-volcaniclastic mudstones comprise hemipelagic deposits. Evidence of formation by diagenetic replacement is preserved in bedded chert, chert nodules and chert matrix of BIF. Silica replacement occurred for bedded chert and chert nodules prior to compaction close to or at the interface between sediment and water, whitch according to the authors¹ indicates it is siliceous hardground. During compaction, though prior to burial metamorphism, the BIF chert matrix formed. Sediment from 2 original sources, mudrock, sandstone, breccia, tuffaceous mudstone from a shelf, and BIF from within the realm of the basin, were resedimented. Resedimentation of shelf sediments to fans on the basin floor occurred at times of third-order lowstands. Granular hydrothermal muds, with a composition of  iron-rich smectite and iron oxyhydroxide and siderite, that had been deposited on the sides of submarine volcanoes, following which it was resedimented by density currents. Bottom currents or turbidity currents driven by gravity, the sediment bodies that resulted may have been contourite drifts. According to the authors¹ this study negates the concept that high-frequency precipitation of iron minerals and silica from ambient sea water is recorded by BIF. They postulate that the precursor sediments to BIF originated in a similar manner to hydrothermal iron oxide deposits, the implication being that at least the iron oxyhydroxide particles originated by vent fluids oxygenation by sea water. A relationship between rising to high sea levels, episodes of hydrothermal activity on the sea floor and BIF, is said by the authors¹ to be reminiscent of the link between eustasy and pulses of ridge spreading.

The authors¹ summary of this article

The authors¹ have summarised their paper in 8 points:

1. Study of the Brockman Supersequence has allowed the documentation of depositional sequences, deposits of paired lowstand and condensed section, and parasequences.
2. Lowstand deposits are comprised of siliciclastic turbidites of sand grade, limestone turbidites and mixed siliciclastic-volcaniclastic turbidites, of mud grade, of a distal basin-floor fan. The contrast of the banded iron formation that is present with turbidites derived from a shelf implies that the sediments that were precursors of it were derived from a source within the basin.
3. The condensed sections are comprised of BIF and hemipelagite of mixed siliciclastic-volcaniclastic. The sediments that are precursors to BIF have been interpreted by the authors¹ to be oxide facies hydrothermal muds rich in iron that were deposited on submarine volcano flanks, the muds possibly being granular and not amorphous.
4. Couplets in BIF, that are massive to plane-laminated, are identical to those found in lowstand mudrocks. The authors¹ say this establishes that BIF precursor sediments were deposited by density currents, and they also suggest the sediments must have been transported by bottom currents, or turbidity currents that were gravity-driven, to have been draped across lowstand basin-floor fans.
5. BIF microbanding is density current lamination, that is compacted fine-grained haematite and siderite, or lamina sets are defined by precursor mud paired with chert. The BIF chert matrix is diagenetic, having been developed during burial, though the early replacement by silica of the precursor sediment is recorded in the precompaction chert nodules.
6. According to the authors¹ bedded chert is the best illustration of silica replacement, with all types of lithofacies being replaced. In BIF, density current laminations are preserved in bedded chert in an identical manner to that in chert nodules. Prior to compaction diagenetic silica replacement occurred, bedded chert being indurated before burial compaction. Early diagenetic chert induration at the time of seafloor exposure is indicated by bedded chert intervals being truncated by erosion.
7. The tops of all depositional sequence and parasequences are defined by bedded chert intervals, that they record basin-wide hiatuses is established by their extent being province-wide. The authors¹ suggest analogy is drawn to Phanerozoic hardgrounds by early seafloor replacement and the basin-wide extent, suggesting that intervals of bedded chert are siliceous hardgrounds. According to the authors¹ their study has defined intervals in deep-marine sediments with an enigmatic iron composition, and also breaks in sedimentation that are of a basin-wide extent.
8. A relationship between changing sea level, tectonic setting and magmatism of the sea floor is reflected by several orders of cyclicity to basin filling. A causal relationship between pulsed hydrothermal/volcanic activity and sea levels that are rising to high, is indicated by the dominance of BIF in sections that are condensed.

Sources & Further reading

1. Krapez, B., M. E. Barley, and A. L. Pickard. "Hydrothermal and Resedimented Origins of the Precursor Sediments to Banded Iron-Formation: Sedimentological Evidence from the Early Palaeoproterozoic Brockman Supersequence of Western Australia." Sedimentology 50 (// 2003): 979-1011.