The Triassic–Jurassic boundary in Asturias (northern Spain): Palynological characterisation and facies

Abstract

A palynological, biostratigraphic, sedimentological and sequence stratigraphy study of the Triassic–Jurassic transition in Asturias (northern Spain) was performed by logging and sampling the cores of two boreholes, the Cantavieyo and Vilorteo boreholes. Four lithological units were differentiated and correlated. The lower unit, composed of mudstones and evaporites deposited in coastal lake to subaerial sabkha and distal alluvial environments, correlated in part with the Upper Triassic Keuper facies present in most of western Europe. The middle unit of well bedded carbonates corresponds to the Solis Member of the Gijon Formation. This unit was deposited on a shallow, subtidal to inter- and supratidal carbonate platform influenced by storms and with interbedded distal fan-delta facies. The Triassic–Jurassic boundary has been located within the carbonates of the Solis Member, which contain a Hettangian ammonite, Caloceras pirondii (Reynés), in tempestitic carbonates attributed to the upper part of this unit. The upper unit, the Barzana Member, is composed of mudstones, evaporites and carbonates, respectively, deposited in distal alluvial environments, in supratidal sabkha environments, and on a shallow subtidal to intertidal platform. Overlying the upper unit, or as a time equivalent of the Barzana Member, the Fabares Member is composed of a carbonate breccia with a lutitic matrix, interpreted as formed by the dissolution of the evaporites of the Barzana Member and the collapse of the originally interbedded carbonates and mudstones. A total of 49 palynomorph taxa have been recorded: 20 spore taxa, 24 pollen taxa, 1 acritarch, 2 prasinophytes, and 2 dinoflagellate cysts. Three palynological assemblages (PA) have been distinguished. PA1, which is typically Rhaetian, corresponds to the Rhaetipollis germanicus Zone. PA2 can be Rhaetian and/or Hettangian in age. PA3, which is Hettangian in age, partly corresponds to the Kraeuselisporites reissingeri Zone. These zones have been correlated with similar zones in northwestern Europe. Palaeoecological analysis of the palynomorph assemblages indicates marked palaeofloral renewal, from diversified, Late Triassic xerophilous plants to an impoverished palaeofloral community at the Triassic–Jurassic transition. The latter was composed of a poorly diversified group of conifers and ferns. The arid climate of the Late Triassic was followed by a short, humid event at the beginning of the Hettangian, during which the renewal and important recovery of vascular cryptogams and conifers occurred. The later predominance of xerophilous species in the Hettangian indicates a recovery of arid climatic conditions. After the turnover at the end of the Triassic, and especially during the Hettangian, the recovery of new species is well marked. Of a total of 21 palynomorphs, 7 species did not surpass the base of the Triassic–Jurassic transition, 6 species appeared in this transition, and 22 species appeared in the Hettangian, marking an outstanding phase of recovery for the plants that generated the studied palynomorphs.