by David R. Bridgland
Good morning everyone from one of your geological guides. Alison has invited me to chip in any extra thoughts I have on today’s geological gems, so here goes.
Those who know of my interest in river terraces will not be surprised to learn that I am greatly interested in the travertine terraces at Antalya. These are sheets of calcium carbonate precipitated from springs and seepages, and possibly also in water courses. Some would call this tufa, as it was formed in cool water, but many prefer to use the term tufa only for a calcium carbonate precipitate that remains soft. These sheets form platforms that represent former coastal plains, gently sloping seawards, with steeper slopes between them (fossil cliff lines). At the present coast we can see modern, active cliffs cut into travertine, of course, which helps us imagine this process. The cliffs are not always cut only into the most recent platform, which would also have been the case when some of the fossil ones were formed. There are three main platforms but there is plentiful evidence of more complexity than that (a case of ‘lumpers versus splitters’, really).
My preferred interpretation would be to envisage these terraces as having formed as a response to the fluctuating sea levels of the Pleistocene, in tune with glacial – interglacial climatic fluctuation (low sea level during cold periods, high during warm). Both precipitation and cliff cutting are likely to represent higher-sea-level episodes, however, so there is a big difference here with river terraces, in which much of the sand and gravel deposition took place during colder climates. That might also explain why not even an ardent ‘splitter’ could find as many terraces here as we know there were climate cycles during the Quaternary. An important element that is required to explain the formation of both types of terraces is progressive uplift. Without it we cannot explain coastal terraces in particular, since we know that sea level was not as high as the higher terraces at any time during the Pleistocene. You couldn’t get sea level that high even if you melted all the ice caps – and we know they have persisted throughout. Here in Turkey that uplift might have a tectonic cause, but tectonic activity usually involves disruption such as faulting and tilting. The well preserved travertine platforms are more suggestive to me of what we call ‘regional uplift’ or ‘epeirogenic uplift’, which is required to explain coastal and river terraces in regions (like ours back home) that are not tectonically active. This is a gentle uplift of widespread areas of landscape.
A probable driver for regional uplift is isostasy, which you might know about from glacio-isostasy (the depression of land areas under the weight of glaciation and their uplift after the ice sheets melt). Rather than ice, we think that erosion (unloading) and sedimentation (loading) represent the isostatic influence at work here. Glacio-isostasy is fully reversible, however, whereas the process that drives terrace formation has been progressive. The uplifed areas remain elevated. My colleague and sometimes co-leader Rob Westaway has promoted a theoretical mechanism for positive feedback that prevents the reversal of uplift in response to erosional unloading. This is the inflow of mobile lower crustal material to beneath uplifting areas from areas that are subsiding under the weight of sedimentation. Most (but not all) such subsiding areas are represented by offshore continental crust that is accumulating sediment; oceanic crust does not have a mobile lower layer and neither do the ancient cratons. Indeed, there is a good match between areas lacking terraces (fluvial and marine) and cratonic crust, which supports Rob’s theory.
If you’d like to read more about these ideas I can send a paper in the PGA that I wrote with Rob (2014). For those who’d like to read more about the Antalya travertines I can send a useful (2006) paper about these too. They are linked to below.
Enjoy your final night (for now) in Antalya.