The Marinoan, or "upper glacial" sequence

 

The second, or upper, phase of the Snowball Earth scenario contains what is called the Marinoan glacial sequence, named after the strata at Marino on the South Australian coast just south of Adelaide. Often it has been designated as the Marinoan-Varangerian glaciation.  There have been so many problems brought up with the concept of this being a glaciation that the proponents of the Snowball Earth scenario have attempted to split this more recent "glaciation" event and make it two separate glaciations in order to bolster their hypothesis.

One of the major problems with this sequence or formation is that although we find it around the earth, we find very little diamictite (which, if glacial, would be classified as tillite), which would be expected with massive glaciation.  Instead, rhythmites are present, indicating unobstructed layering.  The thick Port Askaig tillite in Scotland (and its equivalent in Ireland) was thought to be part of the Marinoan sequence until reclassified as Sturtian due to the fact that the actual Marinoan rhythmites were found higher up in the strata.  It is true that in several places around the world there are some diamictites associated with this upper sequence of rocks. However, in South Australia they are localized and are not extensive. In view of the fact that we seem to be dealing with some kind of worldwide event, it makes more sense to consider that these smaller areas of diamictites are the results of localized earth movement.  By the standards of any human being looking on, they would still seem massive, but on a worldwide scale, they would only be local events.

Again, as in the Sturtian sequence, this Marinoan sequence has a carbonate layer of rock as a capping,  It, also, is world wide. In Namibia, this capping is over 300 metres thick. By contrast, in South Australia this carbonate cap is only about 3 metres thick, but in the vicinity of Hallett Cove, south of Adelaide, its thickness can be measured in centimetres. This carbonate cap in South Australia is a dolomite called the Nucaleena Formation, which, despite its thinness, is nonetheless persistent and is therefore an important geological marker. Below the Nucaleena Formation is the Elatina Formation, the predominant stratum that is considered to be Marinoan glacial deposit. This presumed glacial deposit is present around the world under different names, according to local geology.  It is part of this formation  outcropping in the Flinders Ranges, which showed through paleomagnetic studies that it had originated near the equator.   In the Adelaide area, this part of the strata is referred to as the Reynella Siltstone.  This is one piece of evidence that the Snowball Earth theorists use to indicate that the whole globe, including the oceans, must have been ice-bound if glacial strata are found at sea level at the equator.

Then there is the evidence presented by the Whyalla sandstone.  The Whyalla sandstone is immediately west of the Flinders Ranges and is part of the Marinoan sequence discussed in the preceding paragraph.  There is a difference between the kind of sandstone formed from sand under water and sandstone from sand blown by wind.  Underwater, the layers are flattened out.  But windblown standstone shows the higher angles of deposition which result from being blown over a dune and layered on the other side.  The Whyalla sandstone has all the characteristics of windblown sand.  This windblown sand is strong evidence that at least part of the earth was not covered by ice.

In the Elatina Formation, layerings of material can be seen.  Very often this type of layering, called rhythmites, is presumed to be varves.  Varves are laid down by seasonal depositions, and therefore there would be two a year in any sequence of rock.  Often by counting varves, or presumed varves, in rock, an estimate of the age of the rock is given.  This was also the case in the Elatina Formation.  The rhythmites were presumed to be seasonal.  However, as written up in Australian Geologist #117, Dec. 31, 2000, starting on p. 22, in an article entitled “Proterozoic Equatorial Glaciation: Has Snowball Earth a Snowball’s Chance?” the evidence is given and discussed that these rhythmites are not seasonal all, but were rather the result of daily tides.  Therefore the count is two a day, not two a year!  This was also written up in Scientific American of Feb. 1989, starting on p. 11, in an article entitled “Blame it on the Moon”.   The analysis that Williams, the author of the article in Australian Geologist,  performed on this rock unit  indicated a shallow ocean environment affected by daily tides (this is the same strata which seemed to show it was from equatorial regions when tested paleomagnetically).  The reason that this and similar strata around the world are considered glacial is that occasional dropstones are found within it. It is assumed that the dropstones in these rhythmites were rafted by ice. However, other scenarios are possible for the origin of these dropstones and are discussed later.

The following photographs of the Marinoan deposits are also from near Adelaide, South Australia.

 

1

Exposure of part of the Reynella Siltstone Member of the Elatina Formation on The Cove Road, south of Merino Rocks and north of the Hallett Cove railway stations. The rhythmical bedding and laminae are typical of this Formation, as is the predominantly reddish colour. The ballpoint pen gives a size comparison, both above (up to the right) and below. The photograph below shows details of the bedding and laminae of the Reynella Siltstone in the same location as the photo above.

2

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3

Part of a more massive unit of the Reynella Siltstone on the coastal cliff-face between Merino and Hallett Cove.

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4

An outcrop of Reynella Siltstone can be seen along the cliffs in the upper part of the photograph, forming a deep red line on the coast near Hallett Cove. South of this location, the carbonate capping to the sequence, the Nucaleena dolomite, outcrops near the cliff-face, but is very thin. There is very little similarity between these strata and the Sturtian sequence, except the carbonate capping, so a glacial origin need not necessarily be ascribed to this sequence. Even the carbonate cap is generally admitted not to have formed during the glacial conditions. As the local tillite lenses have an alternative explanation as outlined above, the only evidence left for glacial conditions is the occasional dropstones in the Elatina Formation.

 

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