Climatic influence on sediment composition and early diagenesis during the Triassic–Lower Jurassic transition in the Danish Basin

Petrographical, sedimentological and palynological investigations of the Late Triassic sedimentary succession in the Danish Basin bear witness of major climate changes from arid or semi-arid to more humid. This change in... [ view full abstract ]

Petrographical, sedimentological and palynological investigations of the Late Triassic sedimentary succession in the Danish Basin bear witness of major climate changes from arid or semi-arid to more humid. This change in climatic conditions was accompanied by a shift from alluvial and fluvial deposits (Triassic Skagerrak Formation) to paralic and marine deposits (Upper Triassic – Lower Jurassic Gassum Formation). Despite the same sediment source area remained active during the Triassic and Jurassic, the mineralogical composition of the sediments that reached the basin changed markedly. The Skagerrak Formation contains relatively unstable grains, such as Ca-rich plagioclase, albite, amphibole, epidote, hematite, ilmenite besides common rock fragments. This mineral assemblage indicates that the alteration intensity was relatively low in the arid to semi-arid climate. The Gassum Formation is instead characterized by abundant quartz, with K-feldspar being more abundant than albite, with only few rock fragments and mainly stable heavy minerals, such as zircon and rutile. The absence of unstable minerals shows that the weathering intensity was high during the humid climate, which led dissolution of unstable grains in the sediment source area and during transport.
The red colouration of the Skagerrak Formation resulted from early diagenetic precipitation of iron-oxide/hydroxide coatings under the overall oxidising conditions that existed immediately after deposition. Sparse palynological data indicate vegetation dominated by draught tolerant gymnosperms. Rare organic matter created white or green reduction spots with locally different early diagenetic alterations. Evaporation played an important role during the early diagenesis and resulted in precipitation of gypsum and calcite, as caliche.
In contrast, the Gassum Formation was subjected to higher precipitation and stronger intensity of meteoric water flushing trough the sediments under humid climatic conditions. This caused feldspar dissolution and precipitation of kaolinite. During the Rhaetian the humid conditions enabled the establishment of a rich and diverse flora, dominated by taxodiacean/cypressasean and cheirolepidiacean conifers, which together with seed ferns, ferns and minor lycophytes formed extensive forest mire along the north-eastern coast of the basin. The abundance of organic matter and local coal layers created reducing conditions, which promoted the precipitation of pyrite, siderite and/or chlorite.
The mineralogical composition of sandstones is markedly different depending on whether they were deposited under arid to semi-arid or humid climate conditions, and reflect different intensity of weathering in the sediment source area and during transport. In addition, early diagenetic changes have enhanced the primary mineral differences. The sandstone composition reflects the overall climatic changes.