Marine Redox Evolution and Organic Accumulation In an Intrashelf Basin, Ne Sichuan Basin During the Late Permian

The Middle-Late Permian Emeishan large igneous province (ELIP), located in the western margin of Yangtze craton, SW China, is regarded as the result of the impingement of a mantle plume onto the lithosphere. However, little is known about the petrogenesis of Late Permian basalts in Sichuan Basin, which were previously considered to be located outside the ELIP. Here we report new petrographic, major elements, trace elements and isotopic data (Sr-Nd-Pb) for Late Permian basalts in the boreholes from the northwestern Sichuan Basin. These basaltic rocks are characterized by low SiO2 contents (47.17-49.40 wt.%), high TiO2 contents (3.38-4.11 wt.%) and Ti/Y ratio (539-639), moderate total alkalis contents (Na2O+K2O, 3.36-6.01 wt.%) and Mg# values (40.93-46.04), which geochemically resemble the Emeishan high-Ti basalts. These rocks are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREE), and have (La/Yb)N ranging from 9.95 to 11.78, showing that typical oceanic island basalt (OIB)-like normalized patterns. The fractionation of MREE to HREE suggests that the basalts were generated by low degree of partial melting within the garnet stability field. Low initial 87Sr/86Sr ratios (0.70572-0.70676; t=260 Ma), Pb isotopic ratios [206Pb/204Pb(t) (18.062-18.637), 207Pb/204Pb(t) (15.574-15.641), 208Pb/204Pb(t) (38.33-38.98)], and slightly high &#949;Nd(t) values (-0.03 to +1.34) indicate that the magma formed from a deep mantle source that may possibly be a mantle plume and have negligibly been affected by crustal contamination. This inference is further supported by high Nb/U ratios (20.56-25.70), low Th/Nb (0.17-0.19) and Th/Ta ratios (2.77-3.14), and no visible Nb and Ta anomalie. In addition, thermal history reconstruction using paleogeothermal indicators in the study area shows that the Lower Paleozoic to Middle Permian formations experienced an intensive thermal event and abnormal high heat flow value reached 118.0 mW/m2 at the Late Permian, which may be due to the mantle plume magma upwelling. The geochemical and geothermal characteristics all demonstrate that these basalts were probably generated in response to the Emeishan mantle plume. Thus, we conclude that the ELIP may have larger areal extent and has been played an important role on the thermal evolution of source rocks in the Sichuan basin.

Download Full-text

Growth characteristics of Changhsingian (Late Permian) carbonate platform margin reef complexes in Yuanba Gas Field, northeastern Sichuan Basin, China

Geological Journal ◽

10.1002/gj.2463 ◽

2012 ◽

pp. n/a-n/a ◽

Cited By ~ 2

Author(s):

Lei Chen ◽

Yong-Chao Lu ◽

Tong-Lou Guo ◽

Li-Sha Deng

Keyword(s):

Sichuan Basin ◽

Carbonate Platform ◽

Growth Characteristics ◽

Late Permian ◽

Gas Field ◽

Northeastern Sichuan ◽

Platform Margin ◽

Northeastern Sichuan Basin ◽

Carbonate Platform Margin

Download Full-text

A new scale shoal-forming model and natural gas exploration implications in Feixianguan Formation, northwestern Sichuan Basin, China

Energy Exploration & Exploitation ◽

10.1177/01445987211005219 ◽

2021 ◽

pp. 014459872110052

Author(s):

Hao Peng ◽

Cheng Yin ◽

Yuan Zhong ◽

Yue-Ming Yang ◽

Liang Xu ◽

...

Keyword(s):

Sichuan Basin ◽

Large Scale ◽

Genetic Model ◽

Carbonate Ramp ◽

Early Triassic ◽

Late Permian ◽

Large Area ◽

Feixianguan Formation ◽

Platform Margin ◽

Lagoon Environment

As an important exploration target in the Sichuan Basin, the Lower Triassic Feixianguan Formation has been previously interpreted to be deposited in the trough-platform margin system according to the genetic model of classic lithofacies paleogeography. However, this model does not apply to the northwestern Sichuan Basin. Accordingly, this study proposed a new genetic model that highlighted the carbonate ramp based on the outcrop sections, drilling and logging data, and seismic sections. A large-scale regional regression was found to occur in Sichuan Basin at the end of the Late Permian, leading to the evolution of the original Kaijiang-Liangping Trough into a very shallow tidal flat/lagoon environment at the beginning of the Early Triassic. The sedimentary pattern during the Early Triassic was different from that during the Late Permian. A total of six long-term cycles (LSC1-LSC6) were identified within the Feixianguan Formation. The deposition of LSC1-LSC3 witnessed the filling of the trough to a large extent, and the depositional periods of LSC4 and LSC5 were the main shoal-forming periods. The Feixianguan Formation in the study area was speculated to be deposited in the continuous continental carbonate ramp. With the intensified uplifting of the Longmenshan island chain and the Kangdian ancient land on the northwestern margin of the Upper Yangtze Platform, a large number of terrigenous materials were input into the study area from west to east, leading to the continuous eastward migration of the carbonate facies belt during the deposition of the Fei 1 and Fei 2 Members. Meanwhile, large-area inner-ramp shoal bodies were formed during the deposition of LSC5. It became possible to find a giant gas reservoir belt in the Feixianguan Formation of the northwestern Sichuan Basin and the results of this study provide a new complement to the existing classic trough-platform margin shoal-forming model.

Download Full-text