STRATEGIES OF FLUVIAL-LACUSTRINE SEQUENCE STRATIGRAPHIC CORRELATION IN A HALF GRABEN, LOWER PERMIAN LUCAOGOU LOW-ORDER CYCLE, SOUTHERN BOGDA MOUNTAINS, NW CHINA

Rapid facies and thickness changes of fluvial-lacustrine strata in half grabens demand but also challenge sequence stratigraphic correlation. 8 sections of the Lucaogou low-order cycle in an 88-km² area, 0.1 – 4 km apart, were correlated to identify effective strategies. The Lucaogou is defined by graben-wide transgressive and regressive unconformities. Siliciclastic and carbonate facies were grouped into 5 types of high-order cycles (HCs), 0.1-10 m thick, reflecting environmental changes during lake expansion and contraction. Stacking patterns of the type and thickness of HCs define systems tracts and sequences, reflecting large-scale lake expansion-contraction. Lake type, climate, and tectonic movement of provenance and depositional site were interpreted from paleosols, lithology, and depositional environment. 5 long sections are 98, 181, 197, 88, and 70 m thick and contain 66/18, 135/30, 145/41, 44/11, and 34/8 HCs/sequences. HCs, systems tracts, and sequences are correlative in 100s m. But many HCs and systems tracts, and some sequences are difficult to correlate over 1 km.

Sequence stratigraphic grouping of HCs helps interpret various controlling processes. The same climatic, tectonic, and sedimentary processes associated with different lake types produced different types of HCs and sequences at the center and steep and ramp margins. Upsection, changing processes produced HC- and sequence-scale expansion-contraction cycles of variable lateral and vertical facies shifts. Correlation of sequence-scale cycles among fluvial, littoral, deltaic, and profundal facies-dominated sections in context of lake expansion-contraction delineated temporal and spatial shifts of lake center. Next, progressive lateral shift of different types of HCs within an expansion-contraction sequence delineated systematic facies shifts from profundal to fluvial environments at the HC scale. This procedure ensures accurate correlation of sequences, many HCs, and their amalgamation and absence, so as to reconstruct basin-filling history at the systems tracts and sequence scale. A process-response approach utilizing trends of controlling processes and lake expansion-contraction is an effective strategy in fluvial-lacustrine sequence stratigraphic correlation.