THE NATURE OF INTERPRETED ASH BEDS OF THE PERMIAN SALAGOU FORMATION

In order to assess the nature of two previously dated and interpreted ash beds in the Lodeve Basin, it is important to understand their depositional history and mineralogical content to determine whether they are volcaniclastic (>75% volcanic material) or epiclastic (>25% volcanic material). Exceptionally low magnetic susceptibility (< 0.1 x10^-3) of these buff-colored, very fine-grained, laterally continuous beds in the Salagou Formation (Lodeve Basin, France) have characteristics indicative of tuffaceous deposits. The surrounding red mudstone has magnetic susceptibility values between 0.2-0.35 (x10^-3) if not particularly calcareous, and between 0.1-0.2 (x10^-3) on the resistive, calcareous-rich layers. Two of these previously interpreted ash beds (located by GPS-coordinates 43.67736N; 3.35698E and 43.6655N; 3.33481E) were dated to be 284.4 Ma and 283.5 Ma respectively, using U-Pb geochronology, but their interpretation as true ash beds is debated. Samples from these beds, as well as additional similar samples in other localities (GPS-coordinates 43.673924N; 3.337399E and 43.65089N; 3.302538E) were collected for further hand, petrographic microscope and SEM (Scanning Electron Microscope) analysis. In hand sample they appear similar in composition to the surrounding bedrock, which indicate reworked, possibly epiclastic deposits. If the samples are epiclastic in nature, the previously measured zircon ages may not indicate the age of eruption, and deposition of a true ash was a misinterpretation. Thin sections were made of a thick pyroclastic deposit nearby and compared to the thin sections of the samples. With the SEM, oxide ratios were collected of each thin section which indicate a plagioclase-dominant composition. Previous work that documents the framework mineralogy of surrounding red mudstones of the Salagou Formation show that the feldspar content in the red mudstones are also plagioclase dominant. Preliminary evidence suggests that these previously interpreted “tuff” deposits are instead either re-worked epiclastic ash beds, or they are not ash beds at all. These methodologies can be applied in the future for assessment of primary versus epiclastic volcanic deposits.