Basement structures and basement interfaces are obtained by finite-difference and time-term methods using Pg-wave data from two deep seismic sounding (DSS) profiles in the Artush-Jiashi strong earthquake area. The geo...Basement structures and basement interfaces are obtained by finite-difference and time-term methods using Pg-wave data from two deep seismic sounding (DSS) profiles in the Artush-Jiashi strong earthquake area. The geological units differ considerably in basement depth. The basement structures of contact zones between two geological units also vary obviously, which marks the existence of boundary faults. Finally, we make a remark upon the relationship between characteristics of basement structures and seismicity in the Artush meizoseismal area and the Jiashi earthquake swarm area.展开更多
The research of an analysis of aeromagnetic data collected in the middle Benue Trough in north-central Nigeria is presented.A detailed analysis of basement structures is conducted in order to identify regions with hig...The research of an analysis of aeromagnetic data collected in the middle Benue Trough in north-central Nigeria is presented.A detailed analysis of basement structures is conducted in order to identify regions with high hydrocarbon potential that is different from those discovered by earlier researchers.Aeromagnetic data were filtered by using the Butterworth and Gaussian filters,transformed by engaging the reduction to the equator technique,and subsequently enhanced.To estimate magnetic basement depths at various places throughout the basin,the Euler deconvolution depth weighting approach was used.Eleven(11)sub-basins with depths ranging from-2000 m to-8000 m were also identified by Euler’s findings.The sub-basins trend in the NE-SW direction while the average sediment thickness is found to be more than 3 km.The extracted structural features indicate areas like Kadi Blam and Kado areas in the southeastern part and Ogoja and Obudu in the southern part of the study area as regions with high structural densities.These areas coincide with the areas delineated as the sub-basins.The cross-sections generated reveal depressions caused by the action of some tectonic activities in the area.This study identified undulating basement topography believed to be due to tectonic activities as well as five areas that are possible targets for hydrocarbon exploration.展开更多
Thick-skinned contractional salt structures are widely developed in the western Kuqa depression, northern Tarim basin. To understand the mechanisms that govern the development of these structures, physical experiments...Thick-skinned contractional salt structures are widely developed in the western Kuqa depression, northern Tarim basin. To understand the mechanisms that govern the development of these structures, physical experiments are conducted and the results show that they are largely governed by the activities of basement faults and the forming of paleo-uplifts and basement slopes. The model materials in this study are dry sand, vaseline and plasticene （or hard foam）, simulating the suprasalt, salt, and subsalt layers respectively. The experiments show that, due to the activities of basement faults and the forming of the paleo-uplifts, salt bodies usually accumulate and thicken significantly on the middle top of the paleo-uplifts which are constrained by the pre-exiting boundary faults. The development of large-scale thrust faults and salt nappes is favored by the basement slops with larger dips. The experiments also conclude that differential structural deformation could occur between the subsalt and suprasalt layers because of the presence of salt layers. Their geometries and the locations of structural highs are different, despite of the great similarities in the uplifted areas. The pierced salt diapir is not observed in the experiments, which indicates that the contractional shortening does not effectively accelerate the development of the salt diapir.展开更多
Based on the results obtained from Pg wavefront imaging in active source deep seismic sounding, we propose a new ray hit analysis method for high-resolution seismic refraction profile data processing. This method can ...Based on the results obtained from Pg wavefront imaging in active source deep seismic sounding, we propose a new ray hit analysis method for high-resolution seismic refraction profile data processing. This method can be used to further determine possible refraction interface, especially spatial location of basement and its pattern characteristics in complex upper crustal structure region, making data processing for high-resolution refraction profiles more fine. We use this method to study the crystalline basement structure of east part of A＇nyemaqên suite zone at northeast side of Qinghai-Xizang Plateau and the basement patterns as well as its spreading features at the east part of Anemaqen suite zone and its adjacent region were determined.展开更多
The studies on configuration, character/property of the basement of Qiangtang basin is helpful for evaluating petroleum and nature gas resources as well as understanding the basin evolvement. Recently a moderate to hi...The studies on configuration, character/property of the basement of Qiangtang basin is helpful for evaluating petroleum and nature gas resources as well as understanding the basin evolvement. Recently a moderate to high-grade metamorphic gneiss rock was found underlying beneath very low metamorphic Ordovician strata in Mayer Kangri to the north of the central uplift. That fact actually proved existence of the crystalline basement just the distribution and structures of pre-Paleozoic crystalline basement still remain puzzle. In recent years a number of active sources deep seismic profiling, to aim at lithospheric structure of northern Tibet and petroleum resources of the Qiangtang basin, had been conducted that make it possible to image the structure of the basement of the Qiangtang. Near vertical reflection profiles, included those acquired previously and those during 2004 to 2008, have been utilized in this study. By through the interaction process and interpretation between the reflection profiles and the wide-angle profile, a model with the detailed structure and velocity distribution from surface to the depth of 20 km of Qiangtang basin has been imaged.Based on the results and discussions of this study, the preliminary conclusions are as follows： （1） The velocity structure section （~20 kin） that is interactively constrained by the refraction and reflection seismic data reveals that the sedimentary stratum gently lie until 10 km in the south Qiangtang basin. （2） The basement consists of fold basement （the upper） and crystalline basement （the lower）.The fold basement buried at the average depth of 6 km with a velocity of 5.2-5.8 km/s. The shallowest appear at range of the central uplift. The crystalline basement is underlying beneath the fold basement at the average depth of 10 km with a velocity of 5.9-6.0 km/s except near Bangong-Nujiang suture. （3） The high-velocity body at the depth range of 3-6 km of the central uplift is considered as a fragment of the crystalline basement that perhaps was raised by Thermal or deformation. （4） The lower-consolidated fold basement show more affinity of Yangtze block but the crystalline basement seems more approximate to Lhasa terrene in geophysical nature. We have attempted to improve the resolution and reliability by interaction of the active seismic data and prove it effective to image complex basement structure. It will be a potential to process the piggy-back acquisition data and has wide prospects.展开更多
The Tacheng basin has been identified as a Carboniferous basement with a central uplift, sur- rounded by orogenic belts. This identification was based on the comprehensive analysis of field outcrops, regional magnetic...The Tacheng basin has been identified as a Carboniferous basement with a central uplift, sur- rounded by orogenic belts. This identification was based on the comprehensive analysis of field outcrops, regional magnetic and gravimetric data, skeleton seismic profiles, magnetotelluric profiles and drilling data. Here, we present gravimetric and magnetic data analyses of the basement structures of the Tacheng basin and its base formation. We also provide a magnetotelluric profile analysis of the structural features and tectonic framework of basin-mountain patterns. We use local geology, drilling data, and other comprehensive information to document the tectonic framework of the basement of the basin. Small-scale nappe structures are found in the northern basin, whereas stronger and more pronounced thrusting structures are found to the south and east of the basin. The basin is divided into four first-order tectonic units： a central uplift, a northern depression, a southeastern depression and a western depression. In addition, the Emin sag is suggested as a possible reservoir for oil and gas.展开更多
The seismic data obtained from high resolution seismic refraction profile in Jiashi strong earthquake swarm area in Xinjiang, China were further processed with ray hit analysis method and more complete basement interf...The seismic data obtained from high resolution seismic refraction profile in Jiashi strong earthquake swarm area in Xinjiang, China were further processed with ray hit analysis method and more complete basement interface structural characteristics beneath Jiashi strong earthquake swarm area were determined. The results show that there are two clear basement interfaces at the upper crust in Jiashi strong earthquake swarm area. The first one with buried depth ranging from 2.6 km to 3.3 km presents integral and continuous structure, and it appears an inclined plane interface and smoothly rises up toward Tianshan Mountain. The second basement interface with buried depth from 8.5 km to 11.8 km, is the antiquated crystalline basement of Tarim basin. Near the post number of 37 km, the buried depth of the crystalline basement changed abruptly by 2.5 km, which maybe result from an ultra crystalline basement fault. If taking this fault as a boundary, the crystalline basement could be divided into two parts, i.e. the southwestern segment with buried depth about 11.5 km, and the northeastern segment with buried depth approxi-mately from 8.5 km to 9.0 km. That is to say, in each segment, the buried depth changes not too much. The northeast segment rises up as a whole and upheaves slightly from southwest to northeast, which reflects the upper crustal deformation characteristics under the special tectonic background at the northwestern edge of Tarim basin.展开更多
High-resolution airborne-magnetic data acquired over the Yola extension of the northern Benue Trough,northeastern Nigeria,were analyzed to delineate rock distribution,define basement geometry,determine sedimentary thi...High-resolution airborne-magnetic data acquired over the Yola extension of the northern Benue Trough,northeastern Nigeria,were analyzed to delineate rock distribution,define basement geometry,determine sedimentary thicknesses,and highlight structural framework among others for the hydrocarbon prospect evaluation of sub-basins in the extension.Filtered and refined magnetic intensity data were subjected to Analytic Signal,Horizontal and Vertical Tilt Derivatives and Euler deconvolution depth weighting analyses to enhance various geological features requisite for petroleum prospect evaluation.Sedimentary rock units flanked the northeast and south widely occur in the central part with volcanic rock intrusions delineated within the sedimentary terrain.Extracted lineaments(less than 5 km and up to 45 km)were observed to dominantly follow the established Benue Trough and Yola sub-basin evolutionary trend along the NEeSW and ESEeWNW directions,respectively.Three prominent subbasins with sedimentary thicknesses in excess of 3 km and width of more than 60 km,separated by basement highs of mostly volcanic intrusions origin,were identified.The occurrence of relatively thick and extensive sub-basins which are underlain by structurally active basement rocks suggests relatively high geothermal gradient for generation of hydrocarbon from organic matter rich source rocks while the generated oil and gas are preserved within intra-sedimentary structural traps created when the basement fractures were reactivated.展开更多
The important event in Jurassic tectonics in Mongolia was the subduction and closure of the Mongolia-Okhotsk ocean;correspondingly,basin evolution can be divided into two main stages,related to the orogeny and collaps...The important event in Jurassic tectonics in Mongolia was the subduction and closure of the Mongolia-Okhotsk ocean;correspondingly,basin evolution can be divided into two main stages,related to the orogeny and collapse of the orogenic belt,respectively.The developing of Early-Middle Jurassic basins to the north of the ocean resulted from back-arc extension.The fossil sutures,from the China-SE Asia sub-continent to the south of the ocean,were rejuvenated by subduction-related orogeny;in addition,the Yanshanian intra-continental movement occurred.Three Early-Middle Jurassic molasse basins were developed by movement in Inner Mongolia,all of which stretched westwards (or northwards) into Mongolia;therefore,the molasse basins in eastern and southern Mongolia had the same geometric and kinematic features as the basins in the Inner Mongolia.Owing to the collapse of the MongoliaOkhotsk orogenic belt,a group of rift basins developed during the Late Jurassic.In eastern Mongolia,the NE orientated extensional basins were controlled by the neogenic NE-structure.The contemporary basins in southern Mongolia and the neighboring areas in China were constrained by remobilization (inherited activation) of the latitudinal or ENE-directional basement structures.Three stages can be recognized in the evolution of the Early-Middle Jurassic basins after reversal;the basins also experienced four episodes of reformation.展开更多
Two kinds of margin respectively occur in the Ordos Basin during the Middle-Late Triassic(Yanchang Age),one is foreland margin developed under the background of flexural subsidence by thrusting intensively in the sout...Two kinds of margin respectively occur in the Ordos Basin during the Middle-Late Triassic(Yanchang Age),one is foreland margin developed under the background of flexural subsidence by thrusting intensively in the southwest margin,and the other is intracratonic basin margin by stable subsidence in northern and central parts of the basin.The Middle-Late Triassic Yanchang Formation can be divided into four regional third-order sequences,which are separated by gentle angular unconformity or regional erosion surface,made up of lowstand system tract(LST),expanding system tract(EST)and highstand system tract(HST)from lower to upper within a sequence.But there are distinct differences of the sequence framework between the southwest margin and northern and central parts of the basin.The southwest margin develops heavy conglomerate layer and unconformity as a result of orogeny by thrusting,and the intracratonic basin margin by stable subsidence in the northern and central parts grows aggradational sandstone,conglomerate in fluvio-delta system and parallel unconformity.The depositional framework of southwest margin reflects the tectonic evolution from flexural subsidence by thrusting to rebounded uplift.The formation of sequence boundary is related to the resilient uplift and erosion.The sequence stratigraphic framework and depositional system tract configuration in the foreland basin are controlled by structural activity of the fold and thrust belt,and the sequence succession reflects episodic thrusting of the Middle-Late Triassic toward the foreland basin.The sequence evolution in northern and central parts reflects the depositional succession of fluvio-delta system under intracratonic background,composed of coarse-grained sediment in braided channel deposit at the lower,meandering channel deposit in the middle and fine-grained sediment in the flood plain at the upper,dominated by lake level fluctuation.During the deposit of the LST in the intracraton basin,accommodation space is limited,and results in abundant fluvial sediment migration laterally,erosion and transport,forming laterally sandstone composite and aggradational deposit on the alluvial plain,which constitutes specific erosion unconformity boundary.展开更多
基金State Key Basic Research Development and Programming Project Mechanism and Prediction of Strong Continental Earthquake (G1998040702) and Joint Seismological Science Foundation of China (198062). Contribution No. GPB200109 Research Center of Exploration
文摘Basement structures and basement interfaces are obtained by finite-difference and time-term methods using Pg-wave data from two deep seismic sounding (DSS) profiles in the Artush-Jiashi strong earthquake area. The geological units differ considerably in basement depth. The basement structures of contact zones between two geological units also vary obviously, which marks the existence of boundary faults. Finally, we make a remark upon the relationship between characteristics of basement structures and seismicity in the Artush meizoseismal area and the Jiashi earthquake swarm area.
文摘The research of an analysis of aeromagnetic data collected in the middle Benue Trough in north-central Nigeria is presented.A detailed analysis of basement structures is conducted in order to identify regions with high hydrocarbon potential that is different from those discovered by earlier researchers.Aeromagnetic data were filtered by using the Butterworth and Gaussian filters,transformed by engaging the reduction to the equator technique,and subsequently enhanced.To estimate magnetic basement depths at various places throughout the basin,the Euler deconvolution depth weighting approach was used.Eleven(11)sub-basins with depths ranging from-2000 m to-8000 m were also identified by Euler’s findings.The sub-basins trend in the NE-SW direction while the average sediment thickness is found to be more than 3 km.The extracted structural features indicate areas like Kadi Blam and Kado areas in the southeastern part and Ogoja and Obudu in the southern part of the study area as regions with high structural densities.These areas coincide with the areas delineated as the sub-basins.The cross-sections generated reveal depressions caused by the action of some tectonic activities in the area.This study identified undulating basement topography believed to be due to tectonic activities as well as five areas that are possible targets for hydrocarbon exploration.
文摘Thick-skinned contractional salt structures are widely developed in the western Kuqa depression, northern Tarim basin. To understand the mechanisms that govern the development of these structures, physical experiments are conducted and the results show that they are largely governed by the activities of basement faults and the forming of paleo-uplifts and basement slopes. The model materials in this study are dry sand, vaseline and plasticene （or hard foam）, simulating the suprasalt, salt, and subsalt layers respectively. The experiments show that, due to the activities of basement faults and the forming of the paleo-uplifts, salt bodies usually accumulate and thicken significantly on the middle top of the paleo-uplifts which are constrained by the pre-exiting boundary faults. The development of large-scale thrust faults and salt nappes is favored by the basement slops with larger dips. The experiments also conclude that differential structural deformation could occur between the subsalt and suprasalt layers because of the presence of salt layers. Their geometries and the locations of structural highs are different, despite of the great similarities in the uplifted areas. The pierced salt diapir is not observed in the experiments, which indicates that the contractional shortening does not effectively accelerate the development of the salt diapir.
基金National Natural Science Foundation of China (40334040) and Joint Seismological Science Foundation of China (106076)
文摘Based on the results obtained from Pg wavefront imaging in active source deep seismic sounding, we propose a new ray hit analysis method for high-resolution seismic refraction profile data processing. This method can be used to further determine possible refraction interface, especially spatial location of basement and its pattern characteristics in complex upper crustal structure region, making data processing for high-resolution refraction profiles more fine. We use this method to study the crystalline basement structure of east part of A＇nyemaqên suite zone at northeast side of Qinghai-Xizang Plateau and the basement patterns as well as its spreading features at the east part of Anemaqen suite zone and its adjacent region were determined.
基金supported by the National Natural Science Foundation of China (Grant Nos. 40874045 and 41174081)the Special Funds for Sciences and Technology Research of Public Welfare Trades (Grant No.201011042)
文摘The studies on configuration, character/property of the basement of Qiangtang basin is helpful for evaluating petroleum and nature gas resources as well as understanding the basin evolvement. Recently a moderate to high-grade metamorphic gneiss rock was found underlying beneath very low metamorphic Ordovician strata in Mayer Kangri to the north of the central uplift. That fact actually proved existence of the crystalline basement just the distribution and structures of pre-Paleozoic crystalline basement still remain puzzle. In recent years a number of active sources deep seismic profiling, to aim at lithospheric structure of northern Tibet and petroleum resources of the Qiangtang basin, had been conducted that make it possible to image the structure of the basement of the Qiangtang. Near vertical reflection profiles, included those acquired previously and those during 2004 to 2008, have been utilized in this study. By through the interaction process and interpretation between the reflection profiles and the wide-angle profile, a model with the detailed structure and velocity distribution from surface to the depth of 20 km of Qiangtang basin has been imaged.Based on the results and discussions of this study, the preliminary conclusions are as follows： （1） The velocity structure section （~20 kin） that is interactively constrained by the refraction and reflection seismic data reveals that the sedimentary stratum gently lie until 10 km in the south Qiangtang basin. （2） The basement consists of fold basement （the upper） and crystalline basement （the lower）.The fold basement buried at the average depth of 6 km with a velocity of 5.2-5.8 km/s. The shallowest appear at range of the central uplift. The crystalline basement is underlying beneath the fold basement at the average depth of 10 km with a velocity of 5.9-6.0 km/s except near Bangong-Nujiang suture. （3） The high-velocity body at the depth range of 3-6 km of the central uplift is considered as a fragment of the crystalline basement that perhaps was raised by Thermal or deformation. （4） The lower-consolidated fold basement show more affinity of Yangtze block but the crystalline basement seems more approximate to Lhasa terrene in geophysical nature. We have attempted to improve the resolution and reliability by interaction of the active seismic data and prove it effective to image complex basement structure. It will be a potential to process the piggy-back acquisition data and has wide prospects.
文摘The Tacheng basin has been identified as a Carboniferous basement with a central uplift, sur- rounded by orogenic belts. This identification was based on the comprehensive analysis of field outcrops, regional magnetic and gravimetric data, skeleton seismic profiles, magnetotelluric profiles and drilling data. Here, we present gravimetric and magnetic data analyses of the basement structures of the Tacheng basin and its base formation. We also provide a magnetotelluric profile analysis of the structural features and tectonic framework of basin-mountain patterns. We use local geology, drilling data, and other comprehensive information to document the tectonic framework of the basement of the basin. Small-scale nappe structures are found in the northern basin, whereas stronger and more pronounced thrusting structures are found to the south and east of the basin. The basin is divided into four first-order tectonic units： a central uplift, a northern depression, a southeastern depression and a western depression. In addition, the Emin sag is suggested as a possible reservoir for oil and gas.
基金Foundation item: Joint Seismological Foundation of China (106076)National Natural Science Foundation of China (40474049, 40334040).
文摘The seismic data obtained from high resolution seismic refraction profile in Jiashi strong earthquake swarm area in Xinjiang, China were further processed with ray hit analysis method and more complete basement interface structural characteristics beneath Jiashi strong earthquake swarm area were determined. The results show that there are two clear basement interfaces at the upper crust in Jiashi strong earthquake swarm area. The first one with buried depth ranging from 2.6 km to 3.3 km presents integral and continuous structure, and it appears an inclined plane interface and smoothly rises up toward Tianshan Mountain. The second basement interface with buried depth from 8.5 km to 11.8 km, is the antiquated crystalline basement of Tarim basin. Near the post number of 37 km, the buried depth of the crystalline basement changed abruptly by 2.5 km, which maybe result from an ultra crystalline basement fault. If taking this fault as a boundary, the crystalline basement could be divided into two parts, i.e. the southwestern segment with buried depth about 11.5 km, and the northeastern segment with buried depth approxi-mately from 8.5 km to 9.0 km. That is to say, in each segment, the buried depth changes not too much. The northeast segment rises up as a whole and upheaves slightly from southwest to northeast, which reflects the upper crustal deformation characteristics under the special tectonic background at the northwestern edge of Tarim basin.
文摘High-resolution airborne-magnetic data acquired over the Yola extension of the northern Benue Trough,northeastern Nigeria,were analyzed to delineate rock distribution,define basement geometry,determine sedimentary thicknesses,and highlight structural framework among others for the hydrocarbon prospect evaluation of sub-basins in the extension.Filtered and refined magnetic intensity data were subjected to Analytic Signal,Horizontal and Vertical Tilt Derivatives and Euler deconvolution depth weighting analyses to enhance various geological features requisite for petroleum prospect evaluation.Sedimentary rock units flanked the northeast and south widely occur in the central part with volcanic rock intrusions delineated within the sedimentary terrain.Extracted lineaments(less than 5 km and up to 45 km)were observed to dominantly follow the established Benue Trough and Yola sub-basin evolutionary trend along the NEeSW and ESEeWNW directions,respectively.Three prominent subbasins with sedimentary thicknesses in excess of 3 km and width of more than 60 km,separated by basement highs of mostly volcanic intrusions origin,were identified.The occurrence of relatively thick and extensive sub-basins which are underlain by structurally active basement rocks suggests relatively high geothermal gradient for generation of hydrocarbon from organic matter rich source rocks while the generated oil and gas are preserved within intra-sedimentary structural traps created when the basement fractures were reactivated.
文摘The important event in Jurassic tectonics in Mongolia was the subduction and closure of the Mongolia-Okhotsk ocean;correspondingly,basin evolution can be divided into two main stages,related to the orogeny and collapse of the orogenic belt,respectively.The developing of Early-Middle Jurassic basins to the north of the ocean resulted from back-arc extension.The fossil sutures,from the China-SE Asia sub-continent to the south of the ocean,were rejuvenated by subduction-related orogeny;in addition,the Yanshanian intra-continental movement occurred.Three Early-Middle Jurassic molasse basins were developed by movement in Inner Mongolia,all of which stretched westwards (or northwards) into Mongolia;therefore,the molasse basins in eastern and southern Mongolia had the same geometric and kinematic features as the basins in the Inner Mongolia.Owing to the collapse of the MongoliaOkhotsk orogenic belt,a group of rift basins developed during the Late Jurassic.In eastern Mongolia,the NE orientated extensional basins were controlled by the neogenic NE-structure.The contemporary basins in southern Mongolia and the neighboring areas in China were constrained by remobilization (inherited activation) of the latitudinal or ENE-directional basement structures.Three stages can be recognized in the evolution of the Early-Middle Jurassic basins after reversal;the basins also experienced four episodes of reformation.
基金Supported by the National Basic Research Program of China(Grant No.2003CB214600)
文摘Two kinds of margin respectively occur in the Ordos Basin during the Middle-Late Triassic(Yanchang Age),one is foreland margin developed under the background of flexural subsidence by thrusting intensively in the southwest margin,and the other is intracratonic basin margin by stable subsidence in northern and central parts of the basin.The Middle-Late Triassic Yanchang Formation can be divided into four regional third-order sequences,which are separated by gentle angular unconformity or regional erosion surface,made up of lowstand system tract(LST),expanding system tract(EST)and highstand system tract(HST)from lower to upper within a sequence.But there are distinct differences of the sequence framework between the southwest margin and northern and central parts of the basin.The southwest margin develops heavy conglomerate layer and unconformity as a result of orogeny by thrusting,and the intracratonic basin margin by stable subsidence in the northern and central parts grows aggradational sandstone,conglomerate in fluvio-delta system and parallel unconformity.The depositional framework of southwest margin reflects the tectonic evolution from flexural subsidence by thrusting to rebounded uplift.The formation of sequence boundary is related to the resilient uplift and erosion.The sequence stratigraphic framework and depositional system tract configuration in the foreland basin are controlled by structural activity of the fold and thrust belt,and the sequence succession reflects episodic thrusting of the Middle-Late Triassic toward the foreland basin.The sequence evolution in northern and central parts reflects the depositional succession of fluvio-delta system under intracratonic background,composed of coarse-grained sediment in braided channel deposit at the lower,meandering channel deposit in the middle and fine-grained sediment in the flood plain at the upper,dominated by lake level fluctuation.During the deposit of the LST in the intracraton basin,accommodation space is limited,and results in abundant fluvial sediment migration laterally,erosion and transport,forming laterally sandstone composite and aggradational deposit on the alluvial plain,which constitutes specific erosion unconformity boundary.
