Looking back and ahead on Genetic Sequence Stratigraphy

Key words: 
• genetic sequence stratigraphy /
• looking back /
• looking ahead

Abstract: Genetic sequence stratigraphy on the basis of genetic stratigraphic sequence proposed by Galloway (1989) is a paradigm of Exxon's depositional sequence model. A genetic stratigraphic sequence is the sedimentary product of a depositional episode. The sequence consists of three important components: offlap components, onlap or transgressive 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. The extrabasinal fluvial systems and associated depocenters shift significantly from a genetic stratigraphic sequence to the following sequence. Within each genetic stratigraphic sequence, the paleogeography remains comparatively stable, but depositional styles and patterns between offlap components and onlap or transgressive components change as relative sea level changes.\;Galloway (1990) studied the relationship among Paleogene depositional episodes, genetic stratigraphic sequences, and sediment accumulation rates NW Gulf of Mexico basin. He found that the genetic stratigraphic sequences record episodes of high supply, punctuated by intervals of low supply and consequent transgressive flooding of the basin margin. Within sequences, depositional rates vary with position relative to the contemporaneous shelf margin, with depositional system, and between subbasins. Liu and Galloway (1997) studied Tertiary sediment supply 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 related changes 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 be applied to nonmarine basins. The Qingshankou, Yaojia, and Nenjiang (QYN) formations in the Songliao basin can 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 QYN sequence into a progradational systems tract, lowstand 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 of sequence boundaries. Coals of regional extent require interruption in sediment supply at a basin-wide scale and can bound the sequences.\;High-resolution sequences are also studied in genetic sequence stratigraphy. One example is the sequence analysis of the middle Wilcox subgroup in the Texas coastal plain (Xue and Galloway, 1995). Two high-resolution 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 recognizable because of good lateral continuity whereas unconformities are difficult to identify due to poor lateral continuity, especially 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, which have average durations of 0.8 million years or less. The formation of these sequences is controlled by accommodation or by sediment supply. The significant change seen in fourth-order sequences is the shift in overall regime ratio and reorganization of depositional processes.\;Cyclicity of genetic stratigraphic sequences is another research topic.