Pages

Thursday, September 21, 2017

The Langhe Hills Landscape: The Messinian Salinity Crisis and the Tertiary Piedmont Basin

My previous post treated the Tertiary Piedmont Basin succession through the Tortonian. In this post I treat the sedimentation occurring during the Messinian period.

The Tethys Ocean separated Africa and Europe during the Jurassic and Cretaceous periods but was mostly eliminated as a result of the collision of the continents. Elements of this ocean survive today as the Mediterranean, Black, Caspian, and Aral Seas.
The Mediterranean Sea maintained its connection to the Atlantic and Indo-Pacific Oceans until early in the Miocene when it was reduced with the joining of the two continents along the Middle East front around 14 million years ago (mya). This joining of the two continents began a gradual change to a more arid Mediterranean climate.

The Mediterranean Sea connection to the Atlantic Ocean was maintained through various avenues (see figure below) until the closure of the Rifean Corridor in the early Messinian.


The closure of the Atlantic access precipitated a rapid environmental and climatic change driven by high evaporation rates in the Mediterranean Sea and the inability of riverine sources to replenish the water loss. This event is generally referred to as the Messinian Salinity Crisis and a rough timeline is as follows:

·       5.96 mya – Closure of the Rifean Corridor and partial dessication of the Red Sea
·       5.8 mya – Mediterranean almost dries out. Massive dessication leaves a deep, dry basin 3 to 5 km below sea level with a few hyper-saline pockets
·       5.5 mya – Less dry climatic conditions ensue resulting in more fresh water from the rivers. This fresh water progressively fills the basins and dilutes hyper-saline pockets into larger pockets of brackish water
·       5.33 mya – Zanclean flood. Strait of Gibraltar opens up, quickly filling the Mediterranean with water from the Atlantic Ocean. The end of the crisis.
As described in Progeo Piemonte (Climate variability and past environmental changes: lessons from the Messinian record of the Tertiary Piemonte Basin), “In less than a million years, deep sea sediments are replaced by shallow sea deposits, continental deposits, lacustrine sediments and eventually deep sea sediments.”
According to Nesteroff (The Sedimentary History of the Mediterranean during the Neocene), all of the Messinian deposits they encountered during their drilling explorations in the region “proved to be evaporitic species comprised of dolomitic marls interbedded with massive gypsum, anhydrite and halite.” On land, they found that, in the same period, The Tortonian blue marls were suddenly replaced by either evaporitic series or by lacustrine and continental deposits. The evaporitic deposits are primarily found in the deepest part of the sea but some fragments are found on margins that have been tectonically uplifted.
According to Progeo Piemonte, Messinian age rocks present in the Langhe describe a chronological sequence of the events associated with the Messinian Salinity Crisis:
·       Marls and Mudstones – rocks derived from deep sea sediments. These rocks record the alternation between a warm and humid climate and a cooler, less-humid one. Microfossils in the rock point to the exact moment when the Mediterranean weas cut off from the Atlantic.
·       Gypsum selenite and laminate – These minerals were formed in water with high salinity and point to an increased evaporation linked to the isolation of the Mediterranean Sea.
·       Sandstones and mudstones – Sediments deposited on the continent in low-salinity waters, rich in fossil vertebrate remains and shells of lacustrine molluscs. These remains testify to a savannah environment with temporary pools of fresh water.
·       Calcareous marl – These rocks tell the story of a re-established full connection between the Atlantic and the Mediterranean. These rocks are rich in marine planktonic microfossils recording a deep (around 800 m) marine basin.
·       Erosion surface – This surface describes an event of rapid dismantling of sediments caused by compressive tectonic forces.


©Wine -- Mise en abyme

No comments:

Post a Comment