Origin of hematite in the Middle Cretaceous red chalks on the eastern coast of England

Abstract

Chalks are generally believed to have formed in shallow marine environments. However, the consensus on sources of Fe and origin of hematite in red chalks remains elusive. To better understand the origin of hematite in the Middle Cretaceous red chalks on the eastern coast of England, the evidence has been gathered through diffuse reflection, trace element and iron isotope analysis and combining with the lithologic and stratigraphic characteristics. Rare earth element (REE) patterns exhibit upper-continental-crust-like signatures such as enrichment in light REEs and depletion in heavy REEs. The Fe isotopic composition of the red chalk is typically heavier than that of igneous rocks (except for one sample) and shows negative Eu anomalies (Eu/Eu* = 0.58–0.73), which indicates that the hematite in the red chalk is not of microbial or hydrothermal, but of sedimentary origin. At the same time, the analysis of trace elements suggests that the felsic rocks and sedimentary derivatives are the source of non-carbonate components in the red chalks. The pedogenic features such as mudcracks, secondary white chalks, and calcareous nodules appear in the red chalks, as documented through the field stratigraphic observations. Based on the above evidence and the sedimentary environment of the overlying and underlying layers, it is inferred that the red chalk was deposited in the supratidal environment that had undergone some degree of desiccation and pedogenic modification.