Coal Accumulation in the Lowstand and Highstand Systems T racts in the Huangxian Paleogene Faulted Basin

Key words: 
• lowstand /
• systems /
• tract /
• transgressive /
• systems

Abstract: The Huangxian Paleogene faulted basin is located at the northwestern marg in o f the Jiaobei faulted uplift,the Eastern Shandong faulted block,east to the Tanl u Fracture zone.It is a terrestrial depositional basin,belong to the coal,oil an d gas faulted basin in uplifted block.Two kinds of sequence boundaries are recog nized in the basin filling succession.One is regional tectonic boundary,displayi ng as the regional unconformity.The other is the tectonic-depositional mechanics -transforming boundary,that is the basin water expand or shrink due to the basin tectonics transformation (tectonic stress field change).The upper part of the te ctonic-depositional mechanics boundary is different from the lower part in depos itional association. Three tectonic-depositional cycles can be distinguished in the basin filling suc cession,which were controlled by the activity of the marginal faults of the basi n,According to the tectonic-depositional cycles,the sequences boundaries and the analysis to the evolution of the basin,three sequences can be divided,that is, sequence Ⅰ，Ⅱ and Ⅲ from bottom to top.The sequence Ⅱ and Ⅲ are complete th ird-order sequences,with lowstand,transgressive and highstand systems tracts,con taining coal,oil and gas deposits,Five genetic stratigraphic units can be identi fied.The sequence Ⅰ is mainly fluvial depositional system,forming the first gen etic stratigraphic unit,The lowstand and transgressive systems tracts in sequenc e Ⅱ and Ⅲ constitute four genetic units respectively.The evolution of the faul ted basin was controlled by the tectonic activity in the basin.The lake level ch anges were expressed as integral water expansion or shrink,not in a single direc tion. Study shows that the coal accumulation occurred in both the lowstand and transgr essive systems tracts,stronger in the lowstand systems tracts.The centers of the coal accumulation migrated with the evolution of the sequences.The coal accumul a ted mainly in the transitional belt from the basin's center to margin,also in th e center of the basin.The coal accumulation centers migrated toward the basin's margin in the transgressive systems tracts,with smaller scopes.There was only sm all-scaled coal accumulation in the highstand systems tracts,occurring in the ce nter part of the basin. The activity of the basin's marginal faults was relative ly stable in the lowstand periods.In despite of fluvial-braided river system dev el oped in the basin.the blocks outside the basin uplifted in a small scale.The coa rse sediment supply was not abundant.The lake basin with shallow water beneaped and gradually became swamp,further transmulated to peat swamp,forming large-scal ed coalbeds. The oil shales and the oil-bearing shales in the basin's filling succession cont ain a lot of animal remains and animal fragments distributed in the top of the l owstand systems tracts,which,the authors believe,may be the condensed interval i n the basin's sequence.They are important ore-bearing layers,playing key roles i n the sequence division.