Influence of Climate on the Eearly Diagenesis of Triassic and Jurassic Sediments

Weibel, Rikke; Olivarius, Mette; Friis, Henrik; Nielsen, Lars Henrik; Mathiesen, Anders

The Fennoscandian Shield constitutes the major sediment source area for the sediments in the eastern part of the Norwegian-Danish Basin. Different climate, depositional environments, transport distances and alterations lead to variations in the detrital composition and caused different early diagenetic changes. The proximal Triassic Skagerrak Formation, deposited in the alluvial fans and braided streams, has the highest content of unstable minerals, as heavy minerals, rock fragments and feldspar grains. Only minor alteration of feldspar grains occurred in the arid climate. The Triassic Bunter Sandstone Formation is deposited farther away from the sediments source, though under similar climatic conditions, and has a somewhat lower content of these unstable grains. The Upper Triassic - Lower Jurassic Gassum Formation and the Middle Jurassic Haldager Sand Formation were deposited in fluvial, parallic and shallow marine environments under a humid, wet climate. Under these climatic conditions detrital mineral alteration probably took place in the hinterland as well as in situ the sediment. The Gassum and the Haldager Sand formations therefore contain more stable minerals than the Bunter Sandstone and the Skagerrak formations.

The iron-rich minerals formed in the eogenetic regime vary according to climate and depositional environments, as iron-oxide/hydroxide coatings form in the arid Triassic alluvial fan, ephemeral fluvial, lacustrine and aeolian environment, whereas siderite and pyrite, occasionally as concretions, are characteristic for the humid fluvial, parallic and shallow marine deposits. Concretions in the arid deposits are gypsum/anhydrite or calcite (calcrete), which probably formed by evaporation of groundwater. Kaolin is the dominating authigenic clay mineral in both the Gassum and the Haldager Sand formations. Illite and mixed-layer illite/smectite are the typical authigenic clays in the Skagerrak and Bunter Sandstone formations. These pore-lining clays and iron-oxide/hydroxide coatings cover all detrital grains in the Skagerrak and Bunter Sandstone formations and inhibit authigenic quartz growth, which therefore becomes prismatic or with only few contact points. Pore-filling kaolinite in the Gassum and Haldager Sand formations has less influence on the authigenic quartz growth, which begins on most detrital quartz. Climatic induced early diagenetic variations between the arid and humid deposits thus also affect the burial diagenesis.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013