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Laramide Orogeny

30 April 2021

International Geology Review volume 46 [2004] page 833-838 – the Laramide Orogeny  occurred during Late Cretaceous to Paleocene [80 to 55 million years ago]. You may not that it  encompasses the Cretaceous/Tertiary [or Paleogene] boundar event. One might wonder if the asteroid that was responsible for killing the dinosaurs was also responsible for the Laramide Orogeny, a mountain building episode in North America. It includes the Rocky Mountain fold and thrust belt in Canada and the US. It post dates the Jurassic era – that much is certain. It is also reckoned to be post Early Cretaceous. It stretches, according to some geologists, as far as the Sierra Madre fold and thrust zone in eastern and central Mexico. The article assumes the continental plate was overriding the Pacific ocean plate – but strangley, no evidence of subduction occurring has beed  found. It is a bit of a mystery as it does not seem to conform to Plate Tectonics theory. In spite of that geologists, in general, simply assume subduction was taking place – or the Pacific plate moved horizontally under the continent, without diving at depth. Lots of computer modelling and simulation has been done in order to try and make the evidence fit what can be seen on the ground. The authors add, 'none of the proposed mechanisms for driving Laramide orogenesis satisfactorily explain the geometry of it, the timing or the extent of the orogeny, far removed as it was from the coast [and therefore the assumed point of subduction]. They pose three questions that require resolution – i] existing data suggests that much of Alaska and the Canadian cordillera underwent northward translation [moved northwards] but the existence of the cordillera into the US is unconstrained. We might also ask is it possible there was some sort of pole movement? ii] What was the relationship between northward translation and fold and thrust belt formation? and iii] Why did the Laramide zone occur so far inland, a 1000 km from the coast.

We might add to that – why is there no mention of the K/T boundary event [now known as the K/Pg boundary event]. It seems the Late Cretaceous is getting more and more interesting. No mention of Chicxulub and no mention of the so called inland sea [where the Great Plain is situated, abutting the Laramide]. Or nearly so. It took a while for geologists to accept the idea of a catastrophic event at the end of the Cretaceous, an asteroid strike at the Chicxulub crater in the Yucatan. However, that recognition did not involve an adjustment of geochronology. It is still assumed by most geologists that sediments laid down at this time took place over a long period of time, adhering to the uniformitarian, or gradualist model. If thick layers of sediment can be laid down by a massive volcano, such as the last eruption of Mount St Helens, albeit in a localised fashion, then an asteroid smashing into the earth and burying itself deep in the ground must also have created a lot of sediment [and therefore sedimentary rock formations]. Would it have been as localised as the Mount St Helens event or could it perhaps have been much worse – causing a tidal wave to sweep across central North America and at the same time set in motion the Laramide Orogeny. That is the big question. A catastrophist would argue that yes, it must be possible that some of the sedimentary rocks either side of the K/Pg boundary were laid down rapidly – in days or weeks. In that case we may wonder why geologists have never thought of thinking along the same lines – although we don't know what they think privately. Uniformitarian geochronology still rules the roost. Geology, it seems, is extraordinarily conservative – possibly even reactionary.

In 'The Origin of Mountain Building' [Cliff Ollier and Colin Pain, Routledge:2000], page 96-97, it says that Trimble [in 1980] summarised the history of the region, putting the Laramide event at 70 million years ago [as opposed to the asteroid strike that was later dated at 65 million years ago]. He said it last until 50 million years ago. Trimble went on to compare the uplift to the adjacent plain – which shows no tectonic stress at all. He concluded that vertical uplift of the mountains was unrelated to compromised stress on the basis it did not exist, and could not be seen, on the Great Plains.

The Laramed Orogeny refers to tectonic movements between the Cretaceous and the Tertiary that broke out in a long belt from the Yukon Plateau to the Sierra Madre in Mexico. The Pre Cambrian basalt rock shows little evidence of horizontal displacement but often displays vertical  uplift of many kilometres = the thrusting event.



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