煤形成环境与聚煤盆地的构造活动

Coal forming environments and their relationship to tectonic activity in the Cevennes Stephanian coal basin

Dennis L. Nielson Gamma

The Stephanian C6vennes coal basin is located in the southeastern part of the French Massif centra1．Previous

studies focused on its stratigraphy，sedimentary petrology，coal chemistry ，coal petrology and structural geology.

The aim of this study is to highlight the relationships between the tectonic patterns and the sedimentary environments of coal formations in the late Hercynian interm ontane basins of the Massif central in France , resemble the Tertiary coal basins in China where the occurrence, distribution and the thickness of the coal seams are essentially controlled by early tectonic activity．

1.Regional framework of Cevennes coal basin

The coal measures are up to 2500 m thick and outcrop in the northern part of the basin．Mesozoic strata cover the stephanian series in the southern basin．The two main faults Villefort and Crvennes borde the 50km2 wide coal basin．The basin is divided into a western (study area)and an eastern subbasins．The western subbasin was investigated in this study is divided with three areas，respectively from north to south：La Vernarede graben， Portes horst and La Grand—Combe graben，by the Paulin and Chamarit faults．This paper focuses on the characteristics and distribution of the coal and clastics distribution in the north—western part of the Crvennes coal basin，their palaeoenVimnmentale setting and their relationships with the early tectonic activity．

2.Stratigraphic famework

The Stephanian series of the basin—fill sequence are dominated by detrital rocks including conglomerates or breccias，coarse and fine grained sandstones，sihstones，mudstones and coal seams The depositional sequence canbe divided into six lithostratigraphic units from bottom to top ：① Strriles inf．de La ForSt unit；② Ricard unit；③ Strriles sup．de La Foret unit；④ Grand

—Baume unit；⑤ Luminieres unit；⑥ Champclauson unit．The stratigraphic framework is a result of the distribution of various depositional environments and their mutual relationships Besides the marginal faults，intrabasinal faults such as the Peyraube fault and Malperthus fault also controlled the distribution of units，and the development of asymmetric graben．Th at the distribution of the coarse—grained bodies an d coal seams，in both the lower and the upper parts of the sequence are related to the marginal and intrabasinal faulting．The important mineable coal seams are concentrated in the middle part of the sequence．

3.Coal bearing sequences in the Cevennes basin

The analysis of coal bearing sequences presented here is based on a large amount of geologiical field sections drill cores and well logs complemented by sections in ancient mining galeries．The paleogeographical and paleoen vironmental interpretations were performed using farther inform ation derived from thin section observations and statistical processing of sedimentological data．Correlations within the basin are discussed using poor biostrati graphical data and genetic stratigraphy induced sequences.

3．1 The first stage：deposition of the Strriles inf．de la Foret unit The Strriles inf．de la Forget unit form ed at the beginning of the opening of the sub-basin．Mainly alluvial fan systems started at the center of the La Grand-Combe graben．The near-fan and middle-fan deposits are dominated by breccias，conglomerates and coarse—grained sandstones，containing gneiss and quartz fragments with a max．diameter up to 40 cm．Near-and middle—fan mainly developed in the west part of the study area，close to the marginal Villefort fault and advanced eastward．The far-fan was dominated by sandy and muddy sediments．In the eastern basin alluvial fans are poorly developed，and consist of fine—grained sediments，except close to the Peyraube fault where some small fan conglomerate bodies were deposited．During this stage，the Peyraube fault and together with the western marginal faults，controlled the distribution of the various sediments．

3．2 The second stage：deposition of the Ricard unit

The deposition still occurred in the La Grand-Combe graben，but extended