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
both towards eastwards and west-mentward．The fan conglomerate，well-developed along the western margin，drew back to the Villefort fault，with decreasing of both deposition area and thickness of the series．The far—fan，composed of sandy sediments，developed at the eastern and western margins of the La Grand-Combe graben and widened along a NE—SE direction．In the central part of La Grand-Combe graben，swamps developed in fore—fan plain position，with peat enrichment in the central part of the swamp．The peat enrichment zone is just covering the underlying extra-thick fan-conglomerate of the Strriles inf．de la Foret unit．The corresponding thick coal seams thin rapidly an d pinch out both eastwards and westwards and also split westwards，along the Peyraube fault, which controlled the development of the coal enrichment zone．3．3 The third stage：deposition of the Strriles sup
During this stage，deposition occurred throughout the whole basin，reflecting the increased extent and rate of subsidence．Alluvial fans dominated in the southern La Grand-Combe graben，from the western bo rder and along the eastern margin of the basin，where large fan conglomerates were controlled by the Peyraube fault．Peat swamps settled in the forefan plain．The accumulation of peat，however，was episodically interrupted，resulting in the formation of several thin coal seams with lateral variations in thickness．Some short·lived swamps developed between fans to the south of Champclauson Village，in which very thin coal seams and streaks formed ，while wide-spread deposition of farfan sandy sediments dominated the northern basin of La Grand-Combe graben．To the north in the La Vernarede graben，distinct lithostratigraphic units are recognizable as part of alluvial fan systems．Breccias and conglomerates indicate the near—and middle-fan facies occur in the western part of the sub- basin．The paleocurrent directions indicate that the fans moved towards the central and eastern parts，implying a source of clastics in the western margin．Sandy sediments are representative of the far-fan central part．Swamps developed acrl0ss alluvial fan plains between the Comas and Chamafit faults in the eastern sub-basin．The great number 0f thin coal seams interbedded with clastic bands and pinching out laterally is significant of rapid variations in depositional environment．The Paulin fault at the western margin was active
in the La Vemarede graben，and controlled the spatial distribution of coarse．grmned clastic bodies．The Com as fault at the eastern margin is much less active．1eading to a semi．graben structure.
In the Portes horst,Cofirse-grained clastic bodies also developed in the westem part，and thined eastwards．To the east of Portes village．conglomerates and sandy conglomerates dominated with subordinate sandstones．3．4 The fourth stage：deposition of the Grand-Baume unit
In the La Grand-Combe grabent the alluvial fans are confined to the northeastern part of the sub-basin，with clastics originating from the east.Between the Thrrond and Malperthus faults, extensive river／swamp systems developed．Peat mainly accumulated in alluvial flood plains．The coal enrichment zones formed close to and parallel to the active Malperthus fault．In the La Vemarede graben，the fan bodies are confined to the northern and western parts，while their extent and the grain size of their constituent were reduced，in comparision with the preceding stage． Widely developed swamps mainly replaced far-fan sandy sediments over the alluvial fan plains，leading to a few mineable coal seams．
In the Portes horst，alluvial fan bodies are also confined to the northwestem part，and far-fans developed close to the Portes village．In the peat swamps to the east，some minor thin seams developed．
3．5 The fifth stage：deposition of the Luminieres unit
In the La Grand-Combe graben alluvial fan bodies，tectonically controlled by the Th6rond and the Malperthus faults，developed at the northeastem margin. Conglomerates and gravelly coarse-grained sandstones without large pebbles are dominant in these bodies．In the central part of the graben，swamps with two coal enrichment zones are dominant，surrounded by well sorted arkoses．The swamp and arkose areas(① and② in moved from the southwest to the northeast．In the narrow trough between the Malperthus and Peyraube faults， several mineable coal seams developed．each having an average thickness of less than 1 m． In the La Vemarede graben，near and middle fan bodies are mainly distributed at the northern margin，along the tectonically active Paulin and Com as faults．The western margin was dominated by far—fan fine-grained sandy sediments，with
subordinate coarse sediments．Peat swamps developed in the central and eastem parts，enclosing shallow lacustrine black oil shales with fish fragments and calcareous or ferruginous concretions．Along the western shore of the lake a small but complete delta system developed．Clastics originated from the western margin of the La Vemariede graben．Swamps spread over the alluvial plain in a shallow lake，in which poor minable coal seams formed.
The western margin of the Portes horst was uplifted and emerged，while sandy sedimentation dominated in the central part，and wide swamps in the eastern part，leading to the formation of several thin mineable coal seams． Small alluvial fan groups grew along the eastern margin．
3．6 The sixth stage：deposition of the Ricard unit
In the La Grand-Combe graben deposition occurred only north to Champclauson．While erosion took place to the south．Small alluvial fans developed along the northeastern margin，and fine grained sediments dominated the other depositional regions．Compared with the fifth stage，peatswamps and coal enrichment zones shifted towards the northeast．
The western part of the La Vemarede graben uplifted and no deposits form ed．The triangular shape of the lake formed in the fifth stage changed into an elongated area in which fine-grained arkoses were deposited．Along the western margin，a submerged fan-delta complex prograded into the central lake．This complex is about 100 m thick，and shows a complete reverse-graded bedding．Th e coarser part is composed of coarse．grained sandstones，gravel-bearing coarse-grained sandstones and rounded conglomerates． During this stage，Comas fault was very active while along the Paulin fault activity decreased，resulting in a semi-graben structure，with a steep eastward slope and a gentle westward slope .Well-sorted arkose deposition dominated in the Portes horst．
At the end of the sixth stage，the north-western part of the Crvennes basin uplifted and Stephanian deposition ended
4.Tectonic activity of C6vennes basin
As in other faulted basin．the variations in the intensity and style of the intrabasinal and marginal faults of the Crvennes basin influenced and
controlled the coa1．forming envomments. Taking the La Vemarede graben in the northern basin as an example and based on the study of faults in the La Grand-Combe graben in the southern basin，farther we discuss the evolution of structural framework，the activity of the intrabasin faults and their iniluence on the sedimentary regime and formation of coa1．
It can be seen that during early-middle Stephanian the third deposition stage of the Stieriles sup．de La Foret unit and the fourth deposition stage of the Grand-Baume unit during basin evolution，the Paulin fault at the western margin moved intensely and controlled the spatial distribution of the coarse clastic bodies． At the eastern margin no fault was developed．The structural framework of the basin is a half-graben pattern，with W erbrouck fault having a strike of N125。

．dip of N35。

and dip angle of about 80。

．In the south wall of this fault coarse clastic sediments consisting of sandstone and conglomerate developed．This coarse clastic body shows a lenticular shape thickening towards this fault and runs horizontally parallel to the Werbrouck fault，so clearly it was controlled by the fault．The grain size of this coarse calstic body changed from coarse to fine towards the south，and its rocks from conglomates，sandstones to fine·grained sihstone，mudstone and coal seams．The north wall of this fauh is composed of fine-grained sandstone，sihstone，mudstone and coal seams．
At the end of middle Stephanian and during late Stephanian，i．e．the fifth deposition stage of the Luminieres unit，the near．and middle．fan bodies were distributed on the northern margin of the La Vemarede graben，where the Paulin fault and the Comas fault intersected．During this stage，both east and west marginal faults controlled the stratigraphic framework and resulted in the nearly symmetrical structural framework in the La Vemarede graben．At the end of Stephanian ，i． e．the sixth deposition of the Champclauson unit，fan-delta lobes constructed by coarse sediments were well developed at the region where the Comas fault and the The rond fault．The paleocurrent directions indicate that the fan-delta lobe came from the east．During this stage，the eastern marginal Comas fault moved intensely and the western Paulin fault consumed，resulting in the transform ation of the stmctural framework
of the La Vemarede graben into half-graben with a faulted east slope and a overlying west slope．
5.Conclusions
The detailed structural and sedimentological study of the Stephanian Cevennes coal basin revealed the environmental evolution of the basin and placed the coal forming processes in this context．In the northeastern domain of the Stephanian Cevennes basin，peat swamps developed over the distal part of alluvial fan systems，confined to the basin border in relation to margin faults．Because of relatively intense extra-and-intrabasinal tectonic movements inducing differential subsidence and subsequent rapid clastic supply，peat development and lbreservation were rare．Temporary cessation of the tectonic activity enabled peat development and accumulation over widely spread river systems．
In the La Grand-Combe graben，coal-forming environments underwent an evolution from swamps over alluvial fan systems in the early and late Stephanian stages to swamps over alluvial-lacustrine systems in the middle stage，linked to corresponding three tectonic stages of the basin development and extinction．The distribution and shifting of peat swamps，coal enrichment zones and alluvial deposits were controlled by intrabasinal and marginal faults．The distribution and limits of alluvial fan conglomerate bodies allow to reconstruct easily both the activity of the marginal faults and uplifting of the source regions．On the other hand，reconstruction of intrabasinal fault motion is suggested from the linear and sharp limit between clastic and organic deposits．The tectonic interpretations inferred from depositional characteristics are consistent with structural evolution based on tectonic analyses and the inferred paleo-stress fields and their variations through the basin in space and time．。