A 160 m borehole in the eastern Guadalquivir Basin
An exploration borehole in the eastern Guadalquivir Basin recovered 160 m of core. From top: 0–40 m blue-grey marl with Globorotalia puncticulata and calcareous nannofossils; 40–60 m bioclastic limestone with oysters, pectinids and barnacle plates; 60–75 m gypsum with chicken-wire anhydrite nodules and thin laminated carbonate; 75–110 m red conglomerate with angular schist clasts and no fossils; 110–160 m marl with Globorotalia margaritae and Discoaster. The limestone and gypsum are absent 2 km to the south, where Pliocene marl rests directly on the conglomerate. A company claims the gypsum is a young continental playa deposit and proposes it as a landfill liner source. The regional Messinian salinity crisis is documented nearby. No seismic lines are available.
1. Interpret the five intervals and identify the most likely hiatus or disconformity. Explain your reasoning from fossils, facies and geometry.
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The 110–160 m marl with Globorotalia margaritae and Discoaster is late Miocene open-marine sediment. The 75–110 m red conglomerate with angular schist clasts and no fossils is continental or fan-delta sediment, indicating erosion and proximity to a metamorphic source. The 60–75 m gypsum with chicken-wire anhydrite and laminated carbonate is a restricted marine evaporite, most likely Messinian. The 40–60 m bioclastic limestone with oysters and barnacles is shallow marine, and the 0–40 m marl with G. puncticulata is early Pliocene open marine. The contact between the conglomerate and the overlying gypsum is a flooding surface, while the conglomerate rests on late Miocene marl with a probable erosional hiatus. Laterally, the absence of gypsum and limestone where Pliocene marl rests on conglomerate shows truncation or non-deposition, so the main disconformity is at the base of the Pliocene succession. The borehole alone cannot prove whether the missing section is erosional or a condensed surface; mapping and biostratigraphy are needed.
2. The company claims the gypsum is a young continental playa deposit. What tests would you use to accept or reject that claim, and what is your recommendation for its use as a landfill liner?
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Reject or accept the claim using four independent lines of evidence. First, stratigraphic position: the gypsum lies below Pliocene limestone and above a continental conglomerate, which is consistent with Messinian restriction, not a young playa. Second, facies: chicken-wire anhydrite nodules and laminated carbonate are typical of sabkha or restricted-marine evaporites; a continental playa would more likely show pedogenic features, root traces and variable sulfate minerals. Third, geochemistry: marine Messinian gypsum has a relatively uniform strontium-isotope ratio near 0.7089, whereas continental gypsum is more variable; sulfur isotopes and trace elements can also discriminate sources. Fourth, micropalaeontology and lateral geometry: reworked marine microfossils or an onlapping marine unit would support a marine origin. For landfill use, I would not recommend gypsum. It dissolves, can produce sulfate-rich leachate, and has low mechanical strength; karst or dissolution features could compromise a liner. The company should test permeability, solubility and thickness, and compare with a compacted clay liner. The precautionary recommendation is to reject the gypsum as a liner until those tests are complete.