CHARACTERISTICS AND ANOMALOUS COALIFICATION OF THE TRANSFORMATION FROM THE MIDDLE AND LATE PLEISTOCENE PEAT TO SOFT BROWN COAL IN TENGCHONG BASIN

Abstract: The Tengchong Basin, located in the Western Yunnan, China, is a long and narrow intermountainous basin and about 35kmZin area.The mikkle and laid Pleistocene coal (peat)一bearing strata, made up of two terrigenous sedimentary cycles interbeding extructive rocks, was deposited on the Cretaceous granite base of the basin. Peat of soft brown coal seams occur in the upper section of each cycle. The soft brown coal in the lower cycle was formed about 600, 000 B. P. and the peat in the upper about 75, 000 to 100, 000 B.P., part of which had also been transformed into soft brown coal.Tengchong region lies in the intermediatary zone between Euroasia plate and Indian plate, in which there exsit the remarkable phenomena of geothermal anomaly, the violent and frequet volcanic activities and the frequent tectonic movements of earthquacks during the Quaternary.These poarticular geloogical processes led to the anomalous coaliCication transforming the Quaternary peat into soft brown coal in Tengchong Basin. The evidences in both coal petrology and organic geochemistry show that, in the process of the early coalification in the basin，the physical characteristics and the chemical composition and structure of the organic matter evolvod in certain regularity.With the burial depth increasing and toward the central part of the basin, the compacted degree of peat or brown coal is strengthened; the random reflectance R_r°of euulminites become increasing, ranging from 0.12% of the samples near the groundsurface to 0.33% of those at the depth of about 170 meters; the wavelength .λ_max at the maximum intensity of the tluorescent spectrum under ultraviolet一ligt irradition gradually shifts to the longer wavelengrh, and it should be es pecially noted that there is a distinct turning point between 0. 16% R_r°and 0.22% R_r°，near which the evolving direction of the red/green quotation Q of sporinites changes from decending tendency to astending one with the R_r° iacreasing.As the R_r° of euulrninites rises, there are the increasing tendencies in the calorific value/vumicacid matter ratio, the aromaticity, the fulvic acid/humicacid ratio, the temperature T_max the maximum rate of generation of trotting compounds, the ratio CH₄/nC_2-4and the yield of binocycloaromatic hydrocarbon in cracking compounds and the ratio of aromatic hydrocarbon to heteroaromatic compounds in free compounds, the optical densities E₇₅₀, E₈₀₀, E₁₆₀₀and E₁₃₇₅/E₂₉₂₀ratio, and the concentration Ng of the free radicals in the organic matter, but there exist also the decreasing tendency in the H/C atomic ratio, the S,implying the yield of free hydrocarbon and the Szshowing that of cracking compounds, the yield of monocycloarmatic hyerocarvbon and the carbon pre ference index CPI of n一alkane in cracking hydrocarbon as well as E₁₃₇₅, E₇₂₀, E₁₄₆₅, E₂₈₉₀and E₁₂₆₅, Additionally the absorbed peaks with wave numter 1500-15110cm>in FTIR spectra change from marking to almost disapperring with the Rr rising, and the latter is the typical characteristic of brown coal. It should be yet pointed out that there is a varying rate of evoluation in both geochemical and pitysica: properties.In the burial peat with R°less than 0.16-0.18%and T_max less than of equal to 400℃，the various geochernical properties developed very slowly and dispersed much.In the soft brown coal with R°morc than.0.22%or so and T_maxmore than or equal to 410，they developed at a relatively greater rate and the dispersion became little. These characteristics display that there are still the distinct substages even in the brief early gcochemical coalification from peat to soft brown coal in Tengchong Basin.Basing on these,the authors divide the stage of the early coalitication of the basin into three substages, i.c., those of soft peat, hard peat and T_max，arc used as the principal pa:amcters and the fluorescen: parameters Q and A₂₄₃which arc wonhy to be nurthcr studied, are used as the suppicmcntary those.In addiuon，tthere are also the rcmarkabie differences m the macruscopom physteal properucs amongst the sabstagcp. Through comparing with the Quat nary peat prolilc m Drama Basin. Greece, and the Phccene sost brown coal proflc in Zhcnaa Basin, the Western Yunnan, China, the authors gencralmc the anomaluous characteristics of the early coalification in Tcngchong Basin: 1) the substagc of soft pent.that of normal biogeochcmical coalification, in which there is almost no difference between Tcngchunb Basin and Drama Basin; 2) in the substagc of hard peat, there arc stronger dccompusition, synthctization一condensation polymerization, gclir cation and compaction in Tengchong Basin than to Drama Basin; 3) in the substagc of soft brown coal, there arc the greater rate and the hiahcr dcgrcv o: coalification山Tcngchong Basin than in Zhcnan Bastn.Thus, it can be known that the rate and lhc degree of the early coalification in Tengchong Basin arc much greater than those to the normal casts and the I'au.at there wasa nomalous coalification in Tengchong Basin has been undoubted. The cross places of both NE and N'JV trend faults in the base, naer which the local hight empefra;urc"anomaly occurred, made up the passageways through which the powerful geothermal 11uw、 with the highest degree of coalificauon to the asin tics just in such an anomalous place.Therc exit som; vocauie constituents such as CO₂, F, B and S m the groundu titer in the basin、so it could be dedacel teat the heat given off by the shallow magma bodies be ever the powerful heat of the paiacogeothcrmal field.The anomalous coahncation might be also innmatcly assuctatcd with the and wolcnt actiwnes of earthquakes occurred within and around the basin since the late Pleistocene. 0a the one hand, the impoact wave caused ty the carthauacks frequently compressed the peat or brown coal scams so that the seams occured at the lesser depth display the compacted degree that cou;d be reached lust at the greater depth in the normal cases, and on the other hand, it might also accelerate the coa;tlicatiu, only under the palacogcothermal temperature much lower than that which would be just needed to the normal coalificatiln.Thcrrefore, the authors suggest the concept“anomalous coahlication"，which ineludes the following two aspects: 1) t!ic anomalous rate and degree of coali:ication; 2) the anomaloJs factors of coalilication. So far as Tengchong Basin goes, the factors arc the anomaluosly powerful paiaco-geothermal flows and the violent and frequent tcctomc movements derived from carthquakcs.