Northern Pangea Research Articles

Early and Middle Permian paleoclimates of the Baoshan Block, western Yunnan, China: insight from carbonates

In Permian times the Baoshan Block of western Yunnan, southwest China formed the eastern part of the Cimmerian Continent. Most biogeographical and sedimentological data indicate that the Early Permian Dingjiazhai Formation formed on the block under conditions strongly influenced by the Permo-Carboniferous glaciation. After Early Permian rifting, with post-glaciation climatic amelioration, and as the Baoshan Block drifted northwards to approach South China and Indochina, faunal elements characteristic of Gondwana affinity decreased, while those of Cathaysian affinity increased. Finally, Late Permian faunas are characterized by exclusively Cathaysian elements. This shift of marine provinciality becomes an important indicator in understanding the Permian paleoclimatic evolution of the region. This research investigated the composition of carbonate grain associations and the early diagenetic features of limestones from the upper part of the Dingjiazhai Formation, and from the overlying Yongde and Shazipo formations. A sharp distinction in petrological and diagenetic features is recognized between the Dingjiazhai Formation and the two overlying formations. The Dingjiazhai carbonates are characterized by the bryonoderm (bryozoan-echinoderm)-extended facies of the heterozoan association, with no non-skeletal grains. Because early diagenetic cement was rarely formed, the Dingjiazhai carbonates experienced strong diagenetic compaction. In contrast, the Yongde and Shazipo carbonates show a chloroforam facies of photozoan association, with the common occurrence of non-skeletal grains. These carbonates were well cemented during early diagenetic processes. From comparison with Permian cool-water carbonates from northern Pangea and Tasmania, Australia, the Dingjiazhai carbonates are interpreted as deposits of warm-temperate conditions, while the overlying carbonates are considered to be deposits of subtropical or tropical conditions. This climatic interpretation, based on the petrographic features of the Permian carbonates, agrees well with existing biogeographical data from the region.

Reconstructions of the continents around the North Atlantic at about the 60th parallel

Late Carboniferous–Early Tertiary apparent polar wander (APW) paths (300–40 Ma) for North America and Europe have been tested in various reconstructions. These paths demonstrate that the 500 fathom Bullard et al. fit is excellent from Late Carboniferous to Late Triassic times, but the continental configuration in northern Pangea changed systematically between the Late Triassic (ca. 214 Ma) and the Mid-Jurassic (ca. 170 Ma) due to pre-drift extension. Best fit North Atlantic reconstructions minimize differences in the Late Carboniferous–Early Jurassic and Late Cretaceous–Tertiary segments of the APW paths, but an enigmatic difference exists in the paths for most of the Jurassic, whereas for the Early Cretaceous the data from Europe are nearly non-existent. Greenland’s position is problematic in a Bullard et al. fit, because of a Late Triassic–Early Jurassic regime of compression (>300 km) that would be inherently required for the Norwegian Shelf and the Barents Sea, but which is geologically not defensible. We suggest a radically new fit for Greenland in between Europe and North America in the Early Mesozoic. This fit keeps Greenland ‘locked’ to Europe for the Late Paleozoic–Early Mesozoic and maintains a reconstruction that better complies with the offshore geological history of the Norwegian Shelf and the Barents Sea. Pre-drift (A24) extension amounted to approximately 450 km on the Mid-Norwegian Shelf but with peak extension in the Late Cretaceous.

Secular Variation in Seawater Chemistry During the Phanerozoic As Indicated By Brine Inclusions in Halite

Analysis of data on the chemical composition of primary fluid inclusions (brine inclusions) in primary‐bedded halite from many evaporite formations of northern Pangea shows that during the Phanerozoic the chemical composition of marine brines was oscillating significantly between the Na‐K‐Mg‐Ca‐Cl type and the Na‐K‐Mg‐Cl‐SO4 type. We regard those changes as corresponding to the chemical evolution of the Phanerozoic ocean. The changes correlate in time with earlier suggested secular changes in the mineralogies of marine nonskeletal limestones and potash evaporites. In addition to those secular changes of seawater chemistry, the concentrations of K, Mg, Ca, and SO4 ions in marine brines underwent important changes under the influence of local factors, including rock‐water interaction. These secondary changes did not influence the principal chemical type of the brine characteristic for a given time interval. In the past concentration of the Ca‐ion did not exceed the present concentration in marine water by a...

The permian of northern pangea (vol. 1: Paleogeography, paleoclimates, stratigraphy, vol. 2: Sedimentary basins and economic resources)

The permian of northern pangea (vol. 1: Paleogeography, paleoclimates, stratigraphy, vol. 2: Sedimentary basins and economic resources)

BOOK REVIEWS

Book reviewed in this article:“Tectonics of sedimentary basins”“Peri‐Tethyan platforms”“Petroleum Geochemistry and Geology”“The Permian of Northern Pangea,” Vols 1 and 2Salt Tectonics“Palaeomagnetic applications in hydrocarbon exploration and production”