In order to understand our fieldwork area, Lemps, we must understand the formation of the basin that houses this unique area .The formation of this basin predates to the Triassic.It was during this time where the continental rifting of Laurasia,and the rotation of Gondwana caused major rifting.As rifting occurred, the basin extended , and subsidence occurred.This rifting also lead to the breakup of the supercontinent Pangea, and the opening of the Tethys Sea (Boulila et al, 2010).These events led to more accommodation space , and the deposition of deep marine sediments occurred.
As rifting progressed into the Cretaceous, the following packages composed of marls and limestones formed the stratigraphic makeup of the sediments.These formations are listed in order of decreasing age:
-Oxfordian,Argovian, Kimmeridigian, -Tithonian, -Berriasian,-Valanginian, Hauterivian, Barremian, -Aptian and Albian, -Cenomanian and Turonian.
The following image depicts the location of this basin in present day France.
In the late Jurassic, astronomical cycles of the Earth caused sea level changes, which affected the carbonate productivity and the export of marls into the Vocontian basin (Boulila et al, 2010). Consequently, formations comprised of carbonate-rich marls were deposited pelagically during the Oxfordian, Argovian (locally defined) and the Kimmeridgian geological periods. Interestingly, the production of marls occurred in regressions and colder temperatures, whilst transgressions and warmer climate brought about increased carbonate productivity.
Soon after, sea level rose and a period of transgression occurred during the start of a period the Tithonian.This Tithonian period was rich in limestone production,with nearly no clay sedimentation.Large carbonate breccia deposits were formed due to debris flow and mudflow deposits on tectonically controlled continental slopes, and were even occasionally perturbed by storm wave action(Ferry, Grosheny, 2013).
As time went on, climate oscillations, and Milankovitch cycles prevailed in the Cretaceous and this increased the frequency of sea level changes. As a result, regular alternations between marls and limestones occurred along with deposition of hemipelagic sediments.The formations deposited during this time originate from the Berriasian ,Valanginian, Hauterivian geological periods.
The Barremian geological period occurred next, which actually began with a regression. This regression led to the basin edges moving upwards, and into the photic zone.The abundance of light and ideal temperatures led to reef formation.Following the regression, a period of transgression occurred , and the marine flooding of the basin enabled these reefs to build upwards and into the basin.This upward building is what allowed for thick and hard limestones to form.Turbidity currents and debris flow prevailed too, and thus turbidites and debris flow deposits are also located in the basin.
The end of the Barremian (125 Ma), marks the beginning of the Aptian, a geologic period of regression, where the previously formed reefs were killed.This period was dominated by terrigenous sedimentation and as climate got colder and sea level fell, the carbonate factory from the Barremian halted and hemi-pelagic marls were deposited instead.Besides the marls, sandstones were reworked and deposited as turbidites. The marine ecosystem at this time consisted of ammonites, and shallow sea marine life such as foraminifera, which thrived in the colder temperatures. Additionally, uplift in the western part of the basin led to even lower sea levels(Caillaud et al, 2020).
By the end of the Cretaceous period, a large mass extinction had occurred, followed by the collision of the African and European tectonic plates, which resulted in the disappearance of the western part of the Tethys Ocean, which was previously located between these continents.This marked the start of two collision events in the Miocene and Oligocene respectively. The collision in the Miocene induced N-S dominated thrust faulting, whilst the Oligocene uplift event formed thrust faults in the E-W direction (Smit et al, 2020). The continuous deformation enabled folding to occur, as a consequence of faulting. The fieldwork area of interest Lemps continues to experience deformation,and comprises of a combination of North-South deformation as well as East-West deformation, which explains why the mountains in the area on t follow a preferred orientation, because they have been deformed in multiple orientations..The alpine orogeny however is still ongoing in the Vocontian Basin.
