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 la te Barruelian to early Stephanian B (sensu Carmaux) age; proposals have ear lier been made to base the boundary stratotype for the top of the Barruelia n on a section in the lower half of the Sabero succession. Ten different to nstein bands have been located in the principal coal-bearing lithostratigra phic units: Raposa Formation, Sucesiva Formation, Herrera Beds and the Unic a 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 consti tuent is kaolinite, both as matrix and as abundant fragmentary crystals and phenocrysts, the latter predominantly comprising pseudomorphs after biotit e and feldspar. Graupen and irregular kaolinite masses occur, in some cases conserving relict structures of volcanic glass, Feldspars have been identi fied in the range anorthite-albite; potassic feldspars are in general absen t. 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 Feature s are entirely consistent with a pyroclastic origin. Locally, some apparent ly secondary alteration of kaolinite to illite is observed, and one band sh ows 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 reinterpr etation of Paquete Central, one of the traditionally principal areas of coa l working, previously considered to comprise some five or six seams and now recognised as a series of three or more structural repetitions of two work able seams only; the overall thickness interpreted for the Herrera Beds is correspondingly reduced. The relatively common presence of Ca-plagioclase p henocrysts in some bands suggests a rather more dacitic magmatic origin tha n for the otherwise closely comparable tonsteins of the Massif Central of F rance. The Sabero tonstein bands are closely similar in character and compo sition with those known in the Cinera-Matallana Coalfield and a similar are a of origin is surmised, although further stratigraphical and petrological work is required before firm correlations can be achieved. (C) 2000 Elsevie r Science B.V. All rights reserved.