Category Archives: Sedimentology and stratigraphy

Altyn Tagh range at top - click for detail. Image via Wikipedia

The transport of sediment by wind action is generally visualised as sand dunes of all kind of shapes. Yet shifting sand particles arm strong wind in the manner of a sand blaster so that it can act as an agent of erosion to form peculiar landforms known as yardangs, which often parallel the prevailing wind as linear ridges. Yardangs very rarely form from crystalline rocks, but poorly cemented sedimentary rocks are particularly prone to wind erosion. In a few areas that are very arid it is the dominant sculpting process. One such area is the Qaidam Basin (<50 mm of rain per year) at the northern edge of the Tibetan Plateau. The basin is flanked to the north by the Altyn Tagh mountains, and major passes in that range funnel powerful winds across the basin floor. The yardangs of Qaidam are enormous, reaching up to 50 m high and show clearly on satellite images and often camouflage the trend of bedding in the sedimentary rocks from which they are carved. Formerly thought to be a basin in which sediment was accumulating and being actively folded by tectonic forces related to the India-Asia collision zone, recent work reveals several very surprising aspects of local wind action (Kapp, P. et al. 2011. Wind erosion in the Qaidam basin, central Asia: implications for tectonics, palaeoclimate, and the source of the Loess Plateau. GSA Today, v. 21 (April/May 2011) p. 4-10). Since the Late Pliocene the rate of wind erosion has reached as much as 1 mm per year, so that it is a source of sediment not a repository, to the extent that at least a third of the basin is occupied by exposed folded sediments that wind erosion has exhumed. Yet this is not an area noted for large dust storms.

Yardangs in Quaidam. Image by Joe Zhou via Flickr

The folded sediments are early Pleistocene lacustrine silts and fine sands, which sand blasting has easily sculpted, but many of the yardangs are encrusted with a crust of salt. Indeed several generations of such crusts mark wind-eroded surfaces of different relative ages. It seems that the erosion has occurred in episodes, most likely during cold-dry glacial and stadial periods when the northern jet stream probably shifted south from its present local position around 48°N to the latitude of Qaidam (around 40°N) when the Altyn Tagh’s funnelling effect would have been maximised by prevailing north westerly winds that parallels the yardangs. Such episodes can be shown to have eroded hundreds to thousands of metres of the slowly deforming sediments since about 2.8 Ma. It was at that time that folding began in earnest, and quite possibly the unloading effect of the wind erosion may have assisted the deformation. Where did such vast volumes of sediment end up? Downwind to the south east are the famous loess deposits in the headwaters of the Huang He (Yellow River), whose transport of eroded loess accounts for the great fertility of much of China’s soils and thereby its great carrying capacity for human population. Interestingly, the loess deposits show intricate alternations that match the ups and downs of climate through the late Pleistocene. The link with the Qaidam yardang fields seems convincing

Related articles

• Wind can keep mountains from growing (eurekalert.org)
• Surprise Force: Wind Blew Mountains Away (livescience.com)

At the end of the Miocene tectonic uplift in the region of the present Straits of Gibraltar cut the Mediterranean Sea off from the Atlantic. The only water able to flow into the isolated marine basin was that carried by the major rivers: the Rhône, Danube, Dneiper and Nile. Their volume was exceeded by evaporation, so the Mediterranean became more and more salty, eventually almost drying out completely to leave thick evaporite deposits that still underlie its deepest parts. 5.33 Ma ago, the tectonic barrier was breached so that Atlantic water flooded the whole Mediterranean basin. The Zanclean flood at the start of the Pliocene has been rated as the greatest catastrophic event in the Phanerozoic history of the oceans, but just how dramatic it was has previously only been guessed at. Seismic profiles across and along the line of flooding reveal channels several kilometres across, about 200 km long and up to 250 m deep, now filled with debris (Garcia-Castellanos, D. et al. 2009. Catastrophic flood of the Mediterranean after the Messinian salinity crisis. Nature, v. 462, p. 778-781). Using a well-established model of river incision in mountain rivers, the authors have suggested how the flooding proceeded. From an initial trickle when the original barrier subsided below Atlantic sea level, flow grew exponentially over a few thousand years to about three times that of the modern Amazon discharge (~10⁸ m s^-1), at which rate incision reached more than 0.4 m per day. Around 90% of the Mediterranean basin’s entire volume was flooded in a matter of a few months to two years, sea level rising at up to 10 m per day.

Formation of BIFs halted by Sudbury impact

The peculiar story of banded iron formations (BIFs) is one that ‘runs and runs’, as journalists say. Most of the steel on which North American capital was built comes from gigantic BIF deposits around Lake Superior that formed during the Palaeoproterozoic. Apart from a brief return in the Neoproterozoic, associated with conditions peculiar to ‘Snowball Earth’ conditions, the Superior Province BIFs are the last of any consequence. Most geologists look to a gradual shift in the oxygen content of ocean water as photosynthetic life grew to dominate the Earth after about 2.4 Ga, but the BIFs around Lake Superior turn out to be capped by a blanket of ejecta from a massive extraterrestrial impact that formed the Sudbury Complex (Slack, J.F. & Cannon, W.F. 2009. Extraterrestrial demise of banded iron formations 1.85 billion years ago. Geology, v. 37, p. 1011-1014). But how could even a monstrous bolide have changed ocean chemistry so decisively? John Slack and William Cannon of the US Geological Survey believe that the impact was so violent that it resulted in wholesale mixing of oxygen-bearing surface waters with those of the deep ocean. The evidence they cite is a coincident change in the nature of deep-water hydrothermal deposits from sulfide-bearing to those dominated by iron-oxides.

The Sudbury impact produced a crater around 150 to 270 km across (one of the three largest known on Earth), and it is dominated by remelted basaltic rocks so almost certainly struck the Palaeoproterozoic ocean floor. Its ejected debris probably covered almost 2 million km² and is found up to 800 km from Sudbury, Ontario. Yet, even with impact cavitation and massive tsunamis it seems barely feasible that an impact of a size dwarfed by those of the Lunar surface could completely remix the oceans. However, it is likely that in the Palaeoproterozoic continental crust was gathered together in a supercontinent so that tsunamis could scour much of the surrounding ocean. A plume of vaporised seawater may also have scavenged oxygen from the atmosphere. The evidence seems compelling, and another possibility is that Sudbury was not the only impact site…

And another oddity…

That a major climatic warming occurred at the end of the Palaeocene (55 Ma) is now undoubted, as is its probable cause by emission from the ocean floor of vast amounts of methane. Yet oddly the Palaeocene-Eocene Thermal Maximum (PETM) coincides with a brief geomagnetic reversal 53 ka long (Lee, Y.S. & Kodama, K. 2009. A possible link between the geomagnetic field and catastrophic climate at the Paleocene-Eocene thermal maximum. Geology, v. 37, p. 1047-1050). Both events were short, so a coincidence seems unlikely, in the authors’ opinion. They suggest a connection through the massive power imparted to climatic processes by the PETM (at least a terawatt and perhaps orders of magnitude more), including the deep thermohaline circulation of the oceans that did shift during the event. Had they exceeded a threshold power for circulation of the liquid outer core they may have triggered the brief reversal, which quickly reverted to its previous magnetic polarity. Ths association is not unique, detailed magnetic studies of the K-T boundary event at 65 Ma has revealed a similar short reversal spanning the duration of the iridium peak ascribed to the Chicxulub impact. However, Chicxulub delivered a power of the order a year’s solar radiation in about one second: vastly larger than the climate perturbation of the PETM. Are we seeing here a hidden signal of an extraterrestrial impact behind the methane release? Impacts are no longer as popular as they once were…

Banded iron formations from the late Archaean, Palaeoproterozoic, and in a few short time intervals linked with Neoproterozoic tillites, have long fascinated geoscientists with their counterintuitive occurrence at times when the oceans contained little if any oxygen. Anoxic water allows iron to exist in its Fe²⁺ form, thereby able to dissolve readily. The vast thicknesses and masses of BIFs demands an abundance of mobile iron, but being made predominantly of hematite (Fe₂O₃) their formation requires a balancing superabundance of oxygen. Many geochemists believe photosynthesising blue-green bacteria to have excreted oxygen to oxidise soluble iron to Fe³⁺ and precipitate it as the oxide in shallow water. Yet plenty of BIFs show such delicate banding that deep water is implicated. All the BIF paradoxes would be resolved if another mechanism had caused the oxidation and precipitation of iron. A new clue to what that may have been is the discovery of iron-oxide stromatolites in the monster BIF deposits around Lake Superior (Planavsky, N. et al. 2009. Iron-oxidizing microbial ecosystems thrived in late Paleoproterozoic redox-stratified oceans. Earth and Planetary Science Letters, v. 286, p. 230-242). Iron isotopes and rare earth elements are good indicators of redox conditions, and those in the BIFs indicate anoxic waters, so free oxygen was not available. The stromatolites, however, strongly suggest biogenic precipitation of iron oxide, which is possible through the action of specialist Fe-oxidising bacteria. Indeed, filamentous microfossils occur in the stromatolites. That opens the possibility of BIFs having formed by direct bacterial precipitation in the oxygen-free world before the Great Oxidation Event around 2.2 Ga, in the absence of cyanobacteria.

At a stormy meeting in August 2004at the 32nd International Geological Congress in Florence, a rearguard action was mounted by a group of stalwart geologists to thwart an attempt to expunge the last remnant of the stratigraphic divisions inspired by Giovanni Arduino’s work in the 18th century from the minds of all future geologists (see December 2004 issue of EPN). The Quaternary was under siege. Despite the fact that the International Commission on Stratigraphy (ICS) of the IUGS had already prepared the ground for a coup de gras by stating that, “This composite epoch [the “Quaternary”] is not a formal unit in the chronostratigraphic hierarchy”, its defenders seem to have won (Mascarelli, A.L. 2009. Quaternary geologists win timescale vote. Nature, v. 459, p. 624). The ICS voted on 21 May 2009 to formally define the base of the Quaternary at 2.6 Ma when the Earth began to cool, glaciation began in the Northern Hemisphere and stone tools first appeared in Africa (it was formerly set at 1.8 Ma, for no obvious reason) and to pass that to IUGS for ratification. Another minority group is enraged, with rumours of chewed carpets, as the Quaternary has annexed 800 ka of what previously was designated as Pliocene: ‘It’s kind of a land grab’, commented Philip Gibbard, a Quaternary expert from Cambridge University, possibly with a hint of glee. To me, it is a milestone decision that gives a proper place to tool making, bipedal apes – ourselves – which makes a great deal more sense that the absurd notion of the Anthropocene (see Epoch, Age, Zone or Nonsense? in March 2008 issue of EPN), whose base some deluded colleagues are trying to set at the beginning of the Industrial Revolution!

Early signs of oxygen…but in the wrong place
The so-called ‘Great Oxidation Event’ is marked by the first occurrence of iron-oxide bearing subaerial sediments or palaeosols, widely regarded as occurring at around 2400 Ma. That is probably around the time that photosynthesis overtook the rate of oxidation reactions that previously consumed the oxygen that it produced, so that oxygen could build-up continually in the air. But that date is far earlier than the origin of subaerial photosynthesis and oxygenic photosynthesis must have arisen among oceanic bacteria before then, but only those inhabiting shallow water where the sunlight is. Banded iron formations that go back into the Archaean are often cited as evidence for when such photosynthesis got underway. Their dominant mineral hematite probably formed by oxidation of soluble iron-II and combination of iron-III with free biogenic oxygen, presumed by most workers to be in shallow water. Among the oldest hematite-rich formations is the Marble Bar Chert of Western Australia, dated to 3460 Ma (Hoashi, M. et al. 2009. Primary haematite formation in an oxygenated sea 3.46 billion years ago. Nature Geoscience, v. 2, p. 301-306). The hematite crystals in the chert seem to have formed at above 60ºC in ocean-floor hydrothermal springs that were discharging abundant dissolved iron-II. The authors estimate the basin in which the cherts formed to be between 200 to 1000 m deep. Since at such depths photosynthesis would not be possible, they claim that sufficient oxygen was produced by shallow-water photosynthesis to form oxygenated intermediate and deep ocean waters, reminiscent of far later times in Earth’s history. This is a minority view, and hinges on whether or not the hematite did form directly on the sea floor. One possibility is that it could have been precipitated colloidally from iron-II-rich ocean water in the photic zone where early photosynthesisers would be, to sink to the deeper sea floor. Eventually very fine iron oxide might recrystallise.
See also: Konhauser, K. 2009. Deepening the early oxygen debate. Nature Geoscience, v. 2, p. 241-242.

Much to geologists’ surprise seismic surveys and drilling of the Mediterranean basin revealed that it is floored by an immense thickness of evaporite salts, laid down during the Late Miocene about 6 Ma ago (Messinian Stage).  The event has been dubbed the Messinian salinity crisis, and ascribed to the cutting off from the Atlantic of the Mediterranean Sea causing lowering of sea level by evaporation. The formation of the evaporite sequence has been overshadowed by what happened to restore the Mediterranean: a humongous waterfall at the Straits of Gibraltar. New modelling of the salt-forming event has had a technically surprising outcome (Govers, R. Choking the Mediterranean to dehydration: The Messinian salinity crisis. Geology, v. 37, p. 167-170). It suggests that most of the salt body formed before sea level fell. Sea level lowering reduced the load on the sea floor and allowed isostatic uplift to develop a flow barrier at the Straits of Gibraltar, further cutting off resupply of Atlantic water. The other factor seems to have been the effect of sluggish eastward subduction of a lithospheric slab that eventually resulted in subsidence so that the Atlantic could re-flood the Mediterranean basin.