The pyroclastic tonsteins of the Sabero Coalfield, north-western Spain, and their relationship to the stratigraphy and structural geology

The Sabero Coalfield comprises a succession of strata of over 2000 m, of late Barruelian to early Stephanian B (sensu Carmaux) age; proposals have earlier been made to base the boundary stratotype for the top of the Barruelian on a section in the lower half of the Sabero succession. Ten different tonstein bands have been located in the principal coal-bearing lithostratigraphic units: Raposa Formation, Sucesiva Formation, Herrera Beds and the Unica Beds. Material from over 50 sample locations, from both surface outcrops and from underground coal workings, has been studied by means of microscope thin sections, supported by X-ray diffraction spectrometry (XRD) analysis and whole rock chemical analysis on a small number of selected samples. All of the 10 bands occur as kaolinite coal-tonstein, in which the main constituent is kaolinite, both as matrix and as abundant fragmentary crystals and phenocrysts, the latter predominantly comprising pseudomorphs after biotite and feldspar. Graupen and irregular kaolinite masses occur, in some cases conserving relict structures of volcanic glass, Feldspars have been identified in the range anorthite-albite; potassic feldspars are in general absent. Quartz occurs as acicular splinters and as alpha-form and beta-paraform, with magmatic re-absorption embayments and glass inclusions; tridymite has been identified in two bands. Other accessory primary constituents include idiomorphic apatite and zircon. The composition and petrographical Features are entirely consistent with a pyroclastic origin. Locally, some apparently secondary alteration of kaolinite to illite is observed, and one band shows a lateral progression from kaolinite tonstein, through kaolinite-illite tonstein to illite tonstein, which is apparently coincident with increased intensity of tectonisation. A number of the bands have served as the means of resolving structural and stratigraphic problems in the Sabero Coalfield. Ln particular, the identification of Level 31 has permitted the reinterpretation of Paquete Central, one of the traditionally principal areas of coal working, previously considered to comprise some five or six seams and now recognised as a series of three or more structural repetitions of two workable seams only; the overall thickness interpreted for the Herrera Beds is correspondingly reduced. The relatively common presence of Ca-plagioclase phenocrysts in some bands suggests a rather more dacitic magmatic origin than for the otherwise closely comparable tonsteins of the Massif Central of France. The Sabero tonstein bands are closely similar in character and composition with those known in the Cinera-Matallana Coalfield and a similar area of origin is surmised, although further stratigraphical and petrological work is required before firm correlations can be achieved. (C) 2000 Elsevier Science B.V. All rights reserved.