Barremian-early Aptian charophyte biostratigraphy revisited

Abstract

New Barremian-early Aptian charophyte biozonations are proposed here based on the charophyte succession in the Maestrat Basin (Iberian Chain, Spain). Biostratigraphic analysis distinguished two biozonations, European and Eurasian, which were compared to establish correlations. This is the first time that Early Cretaceous charophyte biozones are calibrated by means of strontium-isotope stratigraphy, enabling their correlation with the coeval marine realm (ammonite biostratigraphy). The European charophyte biozonation is formed of two partial range biozones, whose index species were endemic from the Cretaceous Tethyan Archipelago (present-day Europe and Northern Africa): (1) Globator maillardii var. trochiliscoides (early Barremian) and (2) Ascidiella cruciata-Pseudoglobator paucibracteatus. The base of the A. cruciata-P. paucibracteatus biozone was calibrated with an oyster shell sample whose 87Sr/86Sr is 0.707482, translating to a late early Barremian age. The total time span of this latter biozone is late early Barremian-early Aptian. The Eurasian biozonation involves a novel Lower Cretaceous charophyte biostratigraphy. It has more resolution than the European biozonation, as it is composed of three partial range biozones whose index species were well-distributed in Eurasia. From oldest to youngest these are: (1) Atopochara trivolvis var. triquetra, (2) Hemiclavator neimongolensis var. neimongolensis, and (3) Clavator grovesii var. jiuquanensis. The A. trivolvis var. triquetra biozone characterises the early Barremian, being almost equivalent to the European G. maillardii var. trochiliscoides biozone. The base of the H. neimongolensis var. neimongolensis biozone is marked by the first appearance datum (FAD) of Hemiclavator neimongolensis var. neimongolensis and was calibrated with an oyster shell whose 87Sr/86Sr is 0.707481 corresponding to a late early Barremian age. The top of the H. neimongolensis var. neimongolensis biozone coincides with the FAD of Clavator grovesii var. jiuquanensis. The base of the C. grovesii var. jiuquanensis biozone was dated with an oyster sample sample collected 25 m below the FAD of C. grovesii var. jiuquanensis. The 87Sr/86Sr value of this sample is 0.707489, which translates into an early late Barremian age. Thus, the H. neimongolensis var. neimongolensis biozone spans the late early Barremian-early late Barremian interval. The top of the Clavator grovesii var. jiuquanensis biozone (late Barremian-early Aptian) is marked by the FAD of Clavator grovesii var. lusitanicus. The newly proposed Eurasian charophyte biozonation will facilitate the correlation between non-marine basins in one of the largest continental areas on Earth.