成因层序地层学的回顾与展望

回顾了以成因地层层序为基础的成因层序地层学的形成、发展与研究现状,对成因地层层序及其内部构成、高分辨率成因地层层序、成因地层层序的旋回性、非海相成因地层层序、成因地层层序与沉积物堆积速率等主要观点作了简略评述,并结合我国陆相沉积特征对成因层序地层学未来研究前景作了初步展望。

Looking back and ahead on Genetic Sequence Stratigraphy

Genetic sequence stratigraphy on the basis of genetic stratigraphic sequence proposed by Galloway (1989) is aparadigm of Exxon＇s depositional sequence model. A genetic stratigraphic sequence is the sedimentary product of adepositional episode. The sequence consists of three important components: offlap components, onlap or transgres-sive components, and top and base bounding surfaces reflecting maximum marine flooding. The model of genetic stratigraphic sequence is firstly capplied to the northwest Gulf of Mexico Cenozoic basin,U.S.A. Galloway (1989) recognized nine genetic stratigraphic sequences for the Cenozoic strata of the basin. Theextrabasinal fluvial systems and associated depocenters shift significantly from a genetic stratigraphic sequence tothe following sequence. Within each genetic stratigraphic sequence, the paleogeography remains comparatively sta-ble, but depositional styles and patterns between offlap components and onlap or transgressive components changeas relative sea level changes. Galloway (1990) studied the relationship among Paleogene depositional episodes, genetic stratigraphic se-quences, and sediment accumulation rates NW Gulf of Mexico basin. He found that the genetic stratigraphic se-quences record episodes of high supply, punctuated by intervals of low supply and consequent transgressive floodingof the basin margin. Within sequences, depositional rates vary with position relative to the contemporaneous shelfmargin, with depositional system, and between subbasins. Liu and Galloway (1997) studied Tertiary sedimentsupply to the North Sea basin. They concluded that all episodes of Tertiary sedimen supply correlate to source-terrain tectonic pulses. The history of changing source - area relief and resulting topographic grades and relatedchanges in sediment yield into the basin was a principal control on North Sea Cenozoic sequence development. The precepts of genetic sequence stratigraphy were developed from the study of marine basins, but they can beapplied to nonmarine basins. The Qingshankou, Yaojia, and Nenjiang (QYN) formations in the Songliao basincan be used to illustrate the application of sequence analysis in a lacustrine setting (Xue and Galloway, 1993).They proposed that use of subaerial unconformity and transgressive surfaces allows further subdivision of the QYNsequence into a progradational systems tract, iowstand prograding complex, and retrogradational systems tract.Hamilton and Tadros (1994) chose the regional extensive coals as genetic stratigraphic sequence boundaries in coal - bearing strata in the Gunnedah basin, Australia. Regionally extensive coals can exhibit the essential attributes ofsequence boundaries. Coals of regional extent require interruption in sediment supply at a basin - wide scale and canbound the sequences. High- resolution sequences are also studied in genetic sequence stratigraphy. One example is the sequenceanalysis of the middle Wilcox subgroup in the Texas coastal plain (Xue and Galloway, 1995). Two high - resolu-tion gentic stratigraphic sequences of the middle Wilcox were delineated within the interval time span of 1.5 - 2.2 m.y. based on detailed corrlation of approximately 700 well logs. They discussed the issue of high - resolution stratigraphic correlation using well - log data and concluded that maximum flooding surfaces are easily recognizablebecause of good lateral continuity whereas unconformities are difficult to identify due to poor lateral continuity, es-pecially in shelf environment. The ofher example is case study of the Eocene Yegua Formation in Texas Gulf Coast(Meckel and Galloway, 1996). The Eocene Yegua Formation is made up of six fourth- order sequences, whichhave average durations of 0.8 million years or less. The formation of these sequences is controlled by accommoda-tion or by sediment supply. The significant change seen in fourth - order sequences is the shift in overall regime ra-tio and reorganization of depositional processes. Cyclicity of genetic stratigraphic sequences is another research topic. Two depositional cycles have been identi-fied in the Wilcox strata of the Tertiary Gulf of Mexico. Two depositional cycles bounded by maximum floodingevents were characterized by depocenter shifting, shelf- margin progradation, and depositional system evolution,and lasted several million years. Each of the two depositional cycles generally corresponds to a pulse of tectonism insediment source area. Each depositional cycle is divided into four intervals separated by regional flooding events.The product of each interval is represented by a parasequence set or high - frequency genetic stratigraphic se-qunece. As stated above, genetic sequence stratigraphic analysis has been applied to marine, lacustrine and coal - bear-ing strata. These examples display a common feature that “starved” sedimentary surfaces are present. The“strayed” surfaces, recorded events of maximum marine or lacustrine flooding, or interruption in sediment supplyat a basin - wide scale, can be chosen to separate sediment fills of basins into several genetic stratigraphic units.However, it is difficult to find such basin-wide “starved” surfaces in bains with multiple, variale sediment sup-plies and alluvial- fluvial basin. In a basin with several sediment supply areas, there are no unified “strayed” surfaces if sediment supply con-trols sequence development and several sediment supplies change differently. Thus, there is no basis to use“starved” surfaces to separate sediment fills into genetic stratigraphic units. In alluvial- fluvial basins, there is lack of widely distributed lacustrine mudstones, coal beds, paleosol bedsbecause of very high sediment supply. As a result, it is hard work to recognize “starved” surfaces. Although fluvialchannel stacking patterns respond vertical changes of “base level”, it is difficult to trace and correlate regionally. Future study of genetic sequence stratigraphy combined with nonmarine sedimentary characteristics of China istowards two directions: one is to construct chronostratigraphic framework in basins with no synchronous changesof several sediment supplies, the other is to identify basin - wide “starved” surfaces or substitute surfaces in alluvial- fluvial basins.