7.5.1 Introduction
Sequence stratigraphy has been widely used in various basins and structural settings such as foreland basins, trenches, and island arcs (Walder et al., 1992) , but the research on sequence stratigraphy of carbonate slopes, especially gravity flow sedimentation, is relatively weak. Some data (Cook, 1991; Tose, 1991; Grammer et al., 1992; Glaser et al., 1991; Chiocci, 1992; Trincardi, 1992) believe that gravity flow only occurs during low water levels, and use gravity flow as a criterion. An important symbol for identifying low-water system domains (Tose, 1991; Steinhauff, 1995). However, the sequence stratigraphic study of the Permian carbonate slope in the Youjiang Basin has confirmed that gravity flow can be formed in each period of the sea level rise and fall cycle, and has corresponding internal sequence architecture and genetic framework.
The Youjiang Basin has attracted widespread attention from geologists for a long time due to its special structure, sedimentation, biology, paleogeography, tectonic activity, volcanism and other characteristics as well as its rich mineral resources. And did a lot of work. Due to the influence of the sedimentary structural background, the Permian sequence consists of isolated platform, inter-platform basin and slope deposits, of which the slope facies belt accounts for a considerable proportion (Figure 7.7). As a carbonate platform or isolated platform and Inter-platform basins or special transition zones between deep-water basins, compared with typical continental slopes, have the characteristics of large slopes, relatively shallow water bodies, and developed gravity flows (Walder et al., 1992). They are important for revealing global sea level changes and oil and gas. Exploration is of special significance. Through the study of the Permian slope sedimentary system and sequence characteristics of the Youjiang Basin, this book establishes the slope facies belt, especially the gravity flow sedimentary sequence stratigraphic framework and the corresponding sequence development model.
Figure 7.7 Structural paleogeographic environment of Youjiang Basin
IP—isolated platform; PB—inter-platform basin; OP—open platform
7.5.2 Basin types and evolution
Youjiang Basin refers to southeastern Yunnan, southern Guizhou and most of Guangxi bounded by the Honghe-Jinshajiang Fault, Mile-Shizong Fault, Guiyang-Danchi Fault, and Qinfang Fault . In the early Paleozoic, the Youjiang area was located in the transition zone between the South China quasi-geosyncline and the Yangtze quasi-platform; in the late Paleozoic, the Youjiang area was located in the rift background of the passive margin of the southwestern South China joint plate, showing a rift trough surrounding the distribution of isolated carbonate platforms. Paleogeographic pattern (Figure 7.7). In the Early Permian, due to the rifting of the eastern Paleothys Ocean, it had the nature of a passive continental margin rift, with an overall pattern of alternating platforms and basins. The Qinfang Trough in the southeast corner transformed into a passive continental margin strike-slip basin. At the end of the Early Permian, due to the influence of the Soochow Movement, the Qinfang Trough folds were closed, and the Youjiang Basin, except for the remaining platform basin in the west, was uplifted into a continent. In the Late Permian, the Youjiang Basin entered the back-arc rift basin stage, with a sedimentary pattern of deep-water platform basins surrounding isolated platforms, and the Qinfang area in the southeast corner entered the development period of foreland basins. At the end of the Late Permian, due to the influence of the Jiangsu-Anhui movement, the Youjiang Basin ended its development history as a rift basin in the Late Paleozoic and entered the stage of continental margin activity development.
7.5.3 Slope system
Widely distributed in the strata of various stages of the Youjiang Basin, between the Upper Yangtze shelf platform or Youjiang isolated platform and platform basin or Qinfang deep-water basin Transition zones of varying widths usually form slope sediments with special genetic significance, which are mainly composed of allochthonous sediments, including turbidity currents, debris flows, particle flows, sliding phases, colluvial phases, suspended phases, etc. (Fig. 7.8), and the original mound and reef sediments appear alternately in unequal thickness rhythm or in finger-like contact relationship. Benthic organisms and plankton are mixed, and their distribution is controlled by syngenetic faults of different groups. According to the slope geometry and sedimentary characteristics (Walder et al., 1992), three types of slopes are identified: tumbled (or erosional), slide (or trough) and sedimentary (or aggradational) slopes.
The fall-accumulation type slopes are mainly distributed in the Youjiang back-arc rift basin, and are secondarily common in the Youjiang passive continental margin rift basin in the southeastern Yunnan-Western Guangxi region and the Qinfang passive continental margin strike-slip basin.
It is mainly composed of rockfall or collapse breccia and debris flow breccia. The original rock is platform margin skeleton rock, barrier rock or granular limestone. It transitions to volcaniclastic material and medium-density turbidity current deposits in the basin. and siliceous mudstone, intercalated with high-density calcareous turbidity currents, granular flows, liquefied flows and their transitional sediments, developing reverse grain sequence and various cutting structures. Contour rocks are common, similar to Mario Coniglio (Walder et al., 1992) slope basement skirt and carbonate submarine fan patterns.
Figure 7.8 Genetic sequence of various typical slope facies
A—Suspended phase (Pingle Ertang section); B—Clastic flow phase (Laibin Etou Mountain section); C —Colluvial phase (Hechi Wuwei section); D—turbidite phase (Laibin Taodeng section); E—contourite phase (Guangnan Nasu section) (B and E are based on Walder et al., 1992, with modifications )
Slide-type slopes are mostly found in the central, eastern and northern parts of the Youjiang passive continental margin rift basin, where they develop nodular lime mudstone, striped lime mudstone, pseudobreccia clastic limestone and silica Characterized by high-quality limestone, medium and low-density calcareous turbidites, orthogranular debris flow brecciated limestones and low-density calcareous turbidites are commonly seen, and intra-layer cross-sections, shear zones and slip rock blocks are developed. There are also translational and torsional slip deposits, occasional biomound reefs, and a mixture of in-situ and ex-situ fossil burials, which is equivalent to the fringed platform slope skirt pattern of Mario Coniglio (Walder et al., 1992).
The sedimentary slope is developed in the central and southern part of the Youjiang passive continental margin rift basin. It is composed of medium and low density turbidite, lime mudstone, silty grain marl, and pseudobreccia limestone intercalated with siliceous It is composed of limestone, occasionally intercalated with storm rocks, with common mound and reef deposits, and is dominated by in-situ fossil burial facies, which is similar to the open platform slope model of Mario Coniglio (Walder et al., 1992).
7.5.4 Sequence framework
7.5.4.1 Low-stand system tract
It is the sedimentary product during the period of rapid decline in relative sea level. In a sedimentary slope background, the platform margin and upper slope show exposed scour and erosion surfaces, and even develop small gullies or U-shaped channels, filled with retained gravel or breccia. Calcareous turbidite and clastic debris develop on the lower slope or basin edge. Turbidite (Figure 7.9), at local favorable locations in the late low-water stage, low-water stage biomound reefs develop (Table 7.7, Table 7.8). The corresponding parasequence types mainly include: ① rock avalanche → slump brecciated limestone → liquefied flow deposition → turbidity current deposition → authigenic carbonate wedge; ② debris flow deposition → particle flow deposition → turbidity current deposition → Carbonate progradation complex; ③ rock avalanche → brecciated limestone → debris flow deposition → turbidity current deposition; ④ debris flow deposition → particle flow deposition → turbidity current deposition; ⑤ gravity flow deposition → authigenic carbonate Wedge and carbonate progradation complex; ⑥ Gravity flow deposition → authigenic carbonate wedge → upper slope biomound reef.
Figure 7.9 Typical slope sequence section structure of Wuwei in Hechi, North Guangxi
In a slide-type slope environment, the low-water level system tract has the following main characteristics: ① Slump breccia → Banded lime mudstone → siliceous limestone; ② Nodular lime mudstone → lime mud mound → synovial facies (Chiocci, 1992); ③ Both the matrix part and the clastic part show low compositional maturity; ④ Structure Maturity is low. Matrix and debris support, mixed accumulation of high-density and low-density gravity flow, grain sequence characteristics are not obvious, often interspersed with pelagic suspended phase sediments, indicating that they are products of short-distance transportation and rapid accumulation at the lower part of the slope with a large slope and narrow width; ⑤ middle upper The slope should show a state of erosion and erosion, and sediments are mainly developed on the lower slope; ⑥ In the structural profile, from bottom to top, the erosion surface → weakly graded layers interspersed with suspended phase → suspended phase, and the maturity of the composition gradually increases.
In the fall-deposition slope facies belt, the main manifestations of the low-water level system tract are: ① Large sedimentary thickness, composed of argillaceous siliceous rock, siliceous mudstone and gravity flow deposits; ② Gravity flow sedimentary components complex. There are mainly calcareous rock avalanches, slumps, turbidity currents, debris flows, and particle flow deposits with coal debris on the platform margin, deep-source pyroclastic flow-turbidite deposits and hydrothermal siliceous turbidites, and terrestrial silty turbidites. Sandy muddy turbidite, carbon muddy turbidite and coal dust deposits. The main characteristics and types of parasequences are: ① high-density matrix-supported debris flow deposits → low-density turbidites → suspended volcanic ash flow deposits, with the bottom being an erosion surface; ② silt-bearing muddy turbidites and low-density volcanic ash flow deposits Interbedded calcareous turbidite → siliceous mudstone; ③ volcanic tuffaceous argillaceous siliceous rock → silt-containing mudstone; ④ suspended volcanic ash flow deposition → ash-containing siliceous mudstone → lime mudstone, etc.
7.5.4.2 Shelf edge system tract
The sequence composition, development process and controlling factors of the shelf edge system tract are generally similar to those of the low-stand system tract. However, since the rate of sea level decline during this period is usually lower than the basin subsidence rate, in addition to the exposure of inherited platform margins and intra-platform sedimentary highlands (such as reef shoals), the vast inherited sedimentary depressions, slopes, and platform basin backgrounds on the platform are still submerged by sea water. The exposure time is short, the amount of erosion is small, and the sedimentary facies belt migrates obviously towards the basin. Therefore, compared with the sedimentation during the low water stage, there are also great differences between the shelf edge system tract and the low water table period (Table 7.5). The main manifestations are: ① In the middle and upper Slope facies zone, aggradation-progradation type thick-layer retrograde sedimentary body composed of erosion-filled collapse accumulation → granular limestone → biostratum limestone or mound reef limestone, gradually transforms upward towards the basin edge slope and platform basin. The deepening and fining sedimentary sequence is mostly a thick wedge composed of aggradation-regressive lime mudstone and marl intercalated with low-density calcareous turbidite lenses. The layers are mostly parallel or pseudo-parallel to the lower slope. Toward the platform basin, it is separated from the underlying high-stand gravity flow sedimentary facies by the lithofacies transition surface or scour and erosion surface, and toward the platform it is separated from the overlying transgressive system tract retrogradational parasequence group by the initial flooding surface (Table 7.5 ); ② Gravity flow sedimentation is relatively undeveloped, mainly distal low-density turbidites; ③ During the depositional period of the continental shelf edge system tract, the sea level dropped to near the platform edge, and then began to rise slowly in only a short period of time, and gravity flow Relatively little sedimentation (Trincardi F, 1992).
Table 7.5 Main differences between the shelf edge system tract and the low-stand system tract
7.5.4.3 Transgressive system tract
Slope environmental characteristics during the transgressive period (Steinhauff , 1995) Mainly manifested in: ① tectonic subsidence and rapid relative sea level rise; ② sediment sources include volcanic debris, hemipelagic suspended phase, platform margin materials; ③ sedimentation mainly depends on sea level rise rate, pelagic suspended matter, platform margin margin and heat source supply, thus resulting in a special internal structure of the slope sequence (Table 7.6). However, different regions and different types of slopes have different parasequence characteristics. In sedimentary slopes, the parasequence types of transgressive system tracts mainly include: ①Reformed and redeposited granular limestone or float stone → extremely thin layers of suspended phase and marl interbedded → biological layer limestone or Biological mounds and mud mounds → suspended phase lime mudstone; ② scour and filling of fine gravel lime mudstone → muddy limestone and granular mudstone → suspended phase lime mudstone; ③ volcaniclastic turbidite and calcareous turbidite Rock → radiolarian ash mudstone; ④ volcaniclastic ash mudstone → volcaniclastic turbidite → suspended phase mudstone and mudstone; ⑤ volcanic tuffaceous marl containing suspended phase → ash mud mound → containing radiolarian silicon quality mudstone; ⑥ calcareous debris flow sedimentation → turbidity current sedimentation → suspended phase lime mudstone; ⑦ gravity flow origin slope skirt → biomound → bioherm → sponge spicule gray siliceous rock.
Table 7.6 Carbonate slope sedimentary sequence stratigraphic model
Continued table
In glide-type slopes, it mainly manifests as siliceous gray that thins upward. Mudstone → argillaceous siliceous rock → siliceous rock retrogradational parasequence group (Figure 7.9). Volcaniclastic turbidite develops at the foot of the slope. The upper slope is mainly characterized by small gullies, U-shaped channels and transgressive erosion and filling. The middle slope is interspersed with more seafloor erosion rock blocks, and a mound-shoal reef combination is developed. The top is a thin layer of radiolarian siliceous argillaceous limestone. For example, in the upper part of the slope in central Guangxi, breccia blocks are developed, and the middle slope is partially A mound-shoal reef sequence is developed, with a thin layer of sponge-containing spicule microcrystalline limestone at the top; on the slopes of northern Guangxi, the sequence is mainly composed of siliceous limestone combination or lime mudstone combination, with a thin layer of manganese-containing phosphate silica at the top. Mudstone, rich in planktonic assemblages.
For the trough-type slope, due to the obvious difference in terrain of the trough platform, the slope slope is large, the facies belt is narrow, and it is on the fault trough background with the deep-water platform basin, so the characteristics of the transgressive system tract are similar to those of the deep-water platform. The basins are similar, and the parasequence types mainly include: ① volcaniclastic turbidite → siliceous mudstone with volcanic tuff → siliceous rock with suspended volcanic ash deposits; ② pyroclastic flow and turbidite deposits → argillaceous siliceous rock; ③ Volcaniclastic turbidite and calcareous turbidite → argillaceous siliceous rock and radiolarian mudstone; ④ Sponge-containing spicule marl → mudstone → radiolarian siliceous rock; ⑤ Mudstone → containing manganese, phosphorus, and yellow Iron ore siliceous mudstone → sponge spicule siliceous rock, etc.; ⑥ wollastonite → silica mudstone containing calcareous turbidite lenses → containing deep water relic facies spicule rocks and radiolarian rocks.
7.5.4.4 High-stand system tract
The sequence development in the high-stand period is mainly controlled by: ① tectonic subsidence; ② relative sea level changes; ③ platform margin gravity flow; ④ carbon Salt spin cycle; ⑤ hemipelagic suspension equation (Glaser et al., 1991). In the synovial slope background, the parasequence characteristics are mainly as follows: ① Siliceous spicule lime mudstone → low-density calcareous debris flow and calcareous turbidity current deposits → high-density calcareous debris flow deposits → reef limestone → Reef breccia dolomite, with limonite crust on the top; ② Calcareous turbidite → Debris flow breccia limestone → Mud limestone → Sponge skeleton rock → Reef breccia dolomite; ③ Calcareous turbidite Rock → rock colluvium → biostratal limestone; ④ low-density calcareous debris flow deposits → high-density calcareous debris flow deposits → dolomitic avalanche breccia; ⑤ micritic limestone → granular marl → Calcareous debris flow deposits→mound reef dolomite, etc. In a delta-depositional slope environment, tectonic activity and volcanism tend to be stable and calm during the high-water stage. The development of slope sequences is mainly influenced by platform margin gravity flow, carbonate productivity, hemipelagic sedimentation, trace suspended phase volcanic ash, and deep heat source silicon. Integrated control (Steinhauff, 1995). The main characteristics of the parasequence are: ① silica mudstone → plankton-containing lime mudstone → marl limestone intercalated with calcium turbidite; ② siliceous spicule lime mudstone intercalated with calcium turbidite → marl intercalated with calcium turbidite Flow deposition; ③ rockfall and slump accumulation → particle flow deposition → calcareous turbidite → carbonaceous mudstone turbidite; ④ siliceous rock intercalated with lime mudstone breccia → calcareous debris flow deposition → high-density calcareous debris Turbidite mixed with cuttings and coal dust; ⑤ Silica mudstone mixed with volcanic suspended phase → granular mudstone → biostratum limestone; ⑥ Siliceous sponge ash mudstone mixed with calcium turbidite → muddy limestone → granular ash Rock → accumulation of calcareous rock blocks and breccia; ⑦ Radiolarian marl → supplementary mound and beach combination → progressive reef → mixed reef (Melim et al., 1995), with strong dolomitization at the top ( Figure 7.9).
In the sedimentary slope facies zone, tectonic activity and sea level are relatively stable during the high-water stage. The platform margin facies zone continues to grow, widen and thicken outwards, and the gradually steepening mound-shoal-reef combination becomes the main sedimentation on the slope. Due to the source of material, the rapid outward construction of the platform margin usually leads to the alternate development of sedimentary, synovial and tidal slopes. Different regions and evolutionary periods have different system tract characteristics. But in general, the sedimentary background and parasequence types of sedimentary slope sequences are similar to those of slide-type slopes.
7.5.5 Sequence development model
The sedimentary sequence development of carbonate slopes is a function of the following factors (Grammer et al., 1992; Chiocci, 1992; Schlager, 1986) : ① relative sea level changes; ② platform margin gravity flow; ③ synsedimentary structures; ④ basement topography. Among them, changes in relative sea level directly affect the combination characteristics, geometry, filling sequence, internal architecture and genetic framework of slope sequences (Masetti, 1991; Mullins et al., 1988). Through the study of the slope system and sequence framework of the Youjiang Basin, a comprehensive carbonate slope sequence development model was established (Figure 7.10). The model shows that at different stages of the sea level rise and fall cycle, the environmental physical and chemical conditions are different, and the internal structure and genetic framework of the sequence are different. In the early stage of low water level, the sea level dropped rapidly to the position below the platform edge. The middle and upper parts of the slope - platform edge - platform were exposed to the surface. Only the lower slope and platform basin were submerged underwater. The environment was mainly affected by the relative sea level, basement topography and source properties. influence. Due to the influence of "low sea level" and corresponding platform-platform margin exposed denudation sources (Figure 7.10), muck turbidite composed of intra-basin calcium debris and terrigenous silica debris and a small amount of calcium debris turbidite developed, and Onlap above the platform-basin facies siliceous rock, siliceous mudstone, or siliceous limestone deposits; in the late low-water stage, the relative sea level began to rise slowly from rest (Figure 7.10), overthrowing the middle and upper slopes, and became the environment The main controlling factors are that source sources gradually decrease at the platform edge. Relatively shallow water areas (such as mid-slope) are dominated by biological mudstone or granular limestone, which gradually change into granular mudstone, marlstone and lime mudstone toward the platform basin. Or thick layers of cyclic sediments composed of planktonic phases. In the continental shelf edge area, the relative sea level dropped to near the platform edge, and then began to slowly rise after only a short period of time (Figure 7.10). Therefore, only the intra-platform highlands (such as the mound-reef-shoal combination) and the platform edge were partially exposed to the surface during this period, and the slopes The platform basin is below sea level, and relative sea level and basement topography are the main controlling factors of the sequence. Due to the short exposure time and small amount of erosion during this period, the content of terrigenous silica debris and mixed debris is relatively small compared with the low water level period.
As a result, this period is characterized by in-situ thick layers of planktonic limestone accretion and overburden intercalated with clastic turbidite or calcareous turbidite lenses, with mounds and reefs developing in local favorable locations.
In the early stage of transgression, sea level rose relatively rapidly and overlap toward the platform (Figure 7.10). The main environmental factors include relative sea level, synchronic faults and associated volcanic activities, and pelagic factors (Chiocci, 1992). Because the sea level rise rate in this period usually exceeds the sediment production rate, on the middle and upper slopes, the early low water level or the edge of the continental shelf retreats toward the platform basin and overtakes the middle and upper slopes and platforms exposed to erosion, forming the lower transgressive system tract. It is characterized by planktonic siliceous limestone, radiolarian mudstone, and thin-layered marl interspersed with scoured and retained gravel limestone lenses. Towards the basin edge - lower slope, due to the influence of syngenetic faults, volcanic activity and pelagic factors, the sedimentary combination of volcaniclastic turbidite, planktonic siliceous limestone-siliceous mudstone interbedded with calcareous debris flow is formed. Characteristics: In the late stage of transgression (Figure 7.10), the relative sea level continues to rise rapidly. If the relative sea level rises rapidly due to global sea level rise and tectonic processes, the platform carbonate productivity will become smaller or stop for a long time. , the amount of sediment transported to the outer platform is greatly reduced, and the slope-platform background enters a relatively deep water environment. It is dominated by thin layers of planktonic siliceous limestone and siliceous mudstone intercalated with calcium debris flow and reworked sand deposits, forming Starved slopes or condensed layers with low sedimentation rates (Masetti, 1991); at the end of the transgression or the maximum flooding period, the slope water body is too deep and may be located under the CCD surface. Due to the platform edge source and the slope's own sediment productivity, the At zero, pelagic sediments and volcanic activity were the main controlling factors, resulting in deep-water cherts, spicules, radiolarians, thin layers of deep-water relic facies, and interbedded pyroclastic turbidites.
The early stage of high water level is characterized by a slow rise in relative sea level (Figure 7.10). The productivity of platform-platform margin carbonate rocks is close to the sea level rise rate. The initial accretion of the platform is thickened, and the upper slope is slightly accreted and steepened. , but because the main body of the slope is still in water depth and pelagic factors dominate, it is still characterized by planktonic sediments intercalated with small calcareous turbidite lenses; in the late period of high water level, the sea level is relatively static or slowly declining, and the structure is relatively stable. Environmental physical and chemical conditions are conducive to biological flourishing. The carbonate production rate exceeds the sea level rise rate, causing the maximum rate of slope aggradation and progradation (Glaser et al., 1991; Chiocci, 1992), making the slope overloaded. It is conducive to the continuous construction, accretion and steepening, and gravity collapse of the platform margin. At this time, the slope environment is mainly controlled by the source of the platform margin and the relative sea level. The high-water level sedimentation on the slope appears as S-shaped or inclined sedimentation (Trincardi, 1992), and descends On top of older transgressive deposits. Due to the weakening of seafloor cementation, the gravity flow in this period is characterized by a lithofacies rich in particles and poor in mortar compared with the transgressive period (Mullins et al., 1988), which is specifically manifested in the development of large-scale calcareous gravity flow. It also has a trend from upper slope to platform basin, from colluvial phase to particle flow to debris flow to turbidity current deposition, with the grain size gradually becoming finer and the slope becoming relatively lower; at the end of the high water stage, the platform edge is exposed to the surface, allowing local erosion. Platform margin materials were transported seaward, resulting in the occurrence of pelagic pelagic phase and gravity flow sedimentation of exposed genesis phases.
Figure 7.10 Carbonate slope sequence development model