Triassic Sequence Architecture Of North Carnarvon Basin During Continental Margin Depression Period

During Triassic period, the North Carnarvon basin was a typical gentle continental margin depression basin. Triassic strata is thick layer-shaped and wide distributed. Internal folds deformation in Triassic strata was small, with simple tectonic activity. This paper is focuses on the Mungaroo Formation of the North Carnarvon Basin. Using two dimensional seismic data, well logging, core and paleontological data, based on the global sea level change curve by Haq (1988), referring to the sea shoreline migration cycle curve of Germany and Arctic Circle area, the Mungaroo Formation is divided into two second order parasequences and five third-order sequences. Sequence boundaries are exposure marker interface in the core and transitional surfaces of logging curve from progradation to retrogradation. In the seismic section, the sequence boundary is strong amplitude reflection within the basin, and slight onlap on the edge of the basin.This paper defines the sequence architecture to be the cycle architecture of space and time, and corresponding to sediment filling characteristics in third order sequence framework. Based on the basic unit of third order sequence, sequence architecture is focus on sequence identification and advantages of system tract division, and it can be divided into sequence architecture of cycle architecture and sequence architecture of system tract, according to the research emphasis. This paper analysis sequence architecture mainly based on the base level cycle, accommodation and sediment supply ratios (A/S) of high resolution sequence stratigraphy. This paper defines sequence architecture of cycle architecture to type A (including A1and A2), type B and type C (include C1, C2and C3). It also defines sequence architecture of system tract as type L, T, TH, H (including E-H and H-L). This paper is focus on the analysis of sequence architecture of cycle architecture, within the Mungaroo Formation.Sequence architecture can control the development of sedimentary facies. In the logging facies, the stacked sandbodies which interbeded with some gulf of the distributary channel claystones are very development in proximal delta plain, it is a typical fining upward type A1sequence and the logging curve shows typical box shape; distal delta plain whose sedimentary filling characterized by high stacked degree braided channel sandstone, swamp claystone and crevasse splay sandstone-siltstone are dominated by type A2and C1sequence architecture, and the logging curve shows "bell-box" shape; delta front is dominated by type C2and C3sequence architecture and shows "box-funnel" shape in logging curve. Proximal delta plain is type A1sequence architecture only with rising half cycle with weak amplitude seismic reflection; distal delta plain sequence architecture is A2and C1and the bottom part of distal delta plain seismic facies is weak amplitude reflection, the top part is medium-strong amplitude progradation reflection; delta front which have medium-weak reflection seismic facies characteristic mainly developed C2and C3sequence architecture.According the above responsed relationship of sedimentary facies to sequence architecture, combing with drilling cores and seismic data, we conclude the sedimentary facies of Mungaroo formation of five third-level sequence under the control of the sequence architecture. Research shows that Mungaroo formation is typical shallow water braided river delta with wide delta plain and narrow delta front, and is mainly effected with fluvial process. By analyzing the evolution of Mungaroo delta under the control of third-level sequence architecture, it is believed that the evolutionary phases of Mungaroo delta can be divided into five stages:in the early Carnian, sea-level rise quickly (SQ21), Mungaroo formation was under the early stage of initial development and the area of the delta was relatively large; in the later Carnian (SQ22), sea-level keep rising while the rising rate was slow down, the area of Mungaroo delta was smaller than SQ21; in the early of Norian (SQ31), the sea-level began to drop and the dropping rate was slow, Mungaroo delta was under the expanding period and the area is larger; in the late Norian (SQ32) which sea-lcvcl is the rapidly dropping, Mungaroo delta was under the very development period and the area was the largest; During Rhaetian period (SQ33), North Carnarvon basin suffered whole basin transgression, the rate of sea-level rising was rapid, Mungaroo delta was under the atrophic phase and the area of the delta was the smallest one.Based on the research of sequence architecture of Mungaroo Formation, we establishes a Sequence architecture mode for the glacis continental margin depression basin. The sequence architecture characteristics of Mungaroo delta are mainly the following several aspects:(1) The sequence boundaries shows strong amplitude reflection within the basin, and slight onlap in the edge of the basin. Sequence boundaries are exposure marker interface in the core and transitional surfaces of logging curve from progradation to retrogradation.(2) Seismic facies are parallel and subparallel, medium continuous, medium-high amplitude, and progradation seismic facies are rare. Sedimentary subfacies are different between amplitude and frequency.(3) A/S ratio mainly controls the type of sequence architecture. With the accommodate space increasing, sequence architecture has a change trend from type A to C.(4) The Mungaroo delta system is a asymmetric base level cycle. Delta plain is mainly a rising half cycle which is type A1, A2and C1, which is type L for sequence architecture in system tract. The delta front is a falling half cycle, mainly for type C3and type C2, and type H for sequence architecture in system tract.