Sequence Stratigraphy of Mixed Carbonate and Siliciclastic Sediments in Devonian Along the Western Margin of Yangtze Plate

LI Xiang hui; ZENG Yun fu

There are well exposed carbonate and terrigenous mixtures in the Lower-Middle Devonian in the northwestern Yangtse Craton,China.On the bases of measuring sections,investigating boundary surfaces,and correlating decade sections,hybrid depositional sequences are recognized,and mixings in systems tracts and tectonic significance are analyzed in the paper. Eight hybrid depositional sequences can be distinguished from the top Pragian to the early Givetian in the study area.They are nominated S4,S5,S6,S7,S8,S9,S10,S11 separately which can be classified into two hybrid depositional sequence sets SS2 (S4-S7),and SS3 (S8-S11).Sedimentation in TSTs,Little SMTs and HSTs of depositional sequences in SS2,is characterized by in-situ mixing and punctuated mixing.And source mixings could be only completed in the early TSTs of S7 and S8,but the facies mixing is predominated in early TSTs of most depositional sequences in SS3 and in the HST of S9.It is proposed that mixing of carbonate and terrigenous sediments can be done in anyone of LST/SMT,TST,and HST.This is slightly different from the propositions,among which someone suggested that a mixing could have been only undertaken in HST or in LST. The changes of systems tracts thickness,and relationships between mixing and tectonic are also discussed in the paper.Each of the depositional sequences in SS2 is characterized by the pattern of TST≥HST+SMT except for the S7,and vice versa,any of depositional sequences in SS3 has a texture of HST≥TST+SMT/LST.A relatively active tectonic subsidence was transferred to a stable tectonism at the SMT of S7 in SS2,and a pattern of relatively active to stable tectonism in each of S8 S9,S10,S11 is induced in SS3.That is to say,the patterns of transgression to regression (T-R) could be a way of continuous and slow transgression to relatively fast regression in SS2,and be the tendency of relatively fast transgression to slow regression in each depositional sequence of SS3. We have three questions on the intervals between large-scale boundary surfaces.①If an extensively tectonic activity has ever done at the gap between the pure terrigenous sediments (Pingyipu Formation) and the mixed carbonate and siliciclastic sediments.②If an exposing event existed that is displayed by ooidal hematite between the SS2 and SS3.③If a sea-level falling event with a tectonic uplift took place at the transition of the hybrids to carbonate.

Institute of Sedimentary Geology,Chengdu University of Technology,610059 Chengdu

Received Date: 1998-05-12

Rev Recd Date: 1998-07-30

Publish Date: 1999-09-10

Key words:

Abstract: There are well exposed carbonate and terrigenous mixtures in the Lower-Middle Devonian in the northwestern Yangtse Craton,China.On the bases of measuring sections,investigating boundary surfaces,and correlating decade sections,hybrid depositional sequences are recognized,and mixings in systems tracts and tectonic significance are analyzed in the paper. Eight hybrid depositional sequences can be distinguished from the top Pragian to the early Givetian in the study area.They are nominated S4,S5,S6,S7,S8,S9,S10,S11 separately which can be classified into two hybrid depositional sequence sets SS2 (S4-S7),and SS3 (S8-S11).Sedimentation in TSTs,Little SMTs and HSTs of depositional sequences in SS2,is characterized by in-situ mixing and punctuated mixing.And source mixings could be only completed in the early TSTs of S7 and S8,but the facies mixing is predominated in early TSTs of most depositional sequences in SS3 and in the HST of S9.It is proposed that mixing of carbonate and terrigenous sediments can be done in anyone of LST/SMT,TST,and HST.This is slightly different from the propositions,among which someone suggested that a mixing could have been only undertaken in HST or in LST. The changes of systems tracts thickness,and relationships between mixing and tectonic are also discussed in the paper.Each of the depositional sequences in SS2 is characterized by the pattern of TST≥HST+SMT except for the S7,and vice versa,any of depositional sequences in SS3 has a texture of HST≥TST+SMT/LST.A relatively active tectonic subsidence was transferred to a stable tectonism at the SMT of S7 in SS2,and a pattern of relatively active to stable tectonism in each of S8 S9,S10,S11 is induced in SS3.That is to say,the patterns of transgression to regression (T-R) could be a way of continuous and slow transgression to relatively fast regression in SS2,and be the tendency of relatively fast transgression to slow regression in each depositional sequence of SS3. We have three questions on the intervals between large-scale boundary surfaces.①If an extensively tectonic activity has ever done at the gap between the pure terrigenous sediments (Pingyipu Formation) and the mixed carbonate and siliciclastic sediments.②If an exposing event existed that is displayed by ooidal hematite between the SS2 and SS3.③If a sea-level falling event with a tectonic uplift took place at the transition of the hybrids to carbonate.

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Sequence Stratigraphy of Mixed Carbonate and Siliciclastic Sediments in Devonian Along the Western Margin of Yangtze Plate

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