Palynofacies as sea-level-sensitive proxy in Early Cretaceous marine mudstones

A critical evaluation

authored by

Hauke Thöle, Ulrich Heimhofer, André Bornemann, Jochen Erbacher

Abstract

Stratigraphic distribution patterns of particulate organic matter (POM) have been widely used for facies recognition and paleoenvironmental interpretation as well as to decipher proximal to distal trends within fine-grained sediments. The Lower Cretaceous mudstone-dominated succession in the eastern Lower Saxony Basin (LSB) offers an excellent opportunity to critically evaluate such palynofacies parameters, commonly used to identify transgressive-regressive (T-R) cycles in marine sediments. For the seemingly monotonous succession, a robust sequence stratigraphic framework has been previously established by integrating high-resolution elemental intensity data from X-Ray Fluorescence (XRF) core scanning and biostratigraphy from four drill cores. In this study, the composition and distribution of the POM has been assessed by analysis of 220 strew mounts using transmitted-light microscopy. Overall, the POM composition indicates deposition in a mud-dominated proximal to distal shelf setting. The ratio of opaque versus translucent phytoclasts (OP/TR ratio) shows a distinct long-term increase from the Berriasian onwards with maximum values during the early Hauterivian, followed by a subsequent decrease in OP/TR ratio. This trend broadly reflects the overall T-R evolution of the succession interpreted from Si/Al changes. This also applies to the size and sorting of opaque phytoclasts, with the greatest amplitude changes in opaque particle size parameters being observed in the more proximal deposits of the studied succession. On the other hand, the ratio of terrestrial versus marine palynomorphs (T/M ratio), often applied as indicator of proximal to distal trends and distances from the coastline, shows no correlation with the T-R cycles. Systematic long- and short-term trends visible in T/M ratio correspond to variations in the XRF-derived Ca/Ti stratigraphic trend, which is interpreted to reflect variations in carbonate content. This may indicate that the T/M ratio in the LSB is largely controlled by variations in marine palynomorph flux, probably related to productivity changes of the organic-walled microplankton.

Organisation(s)

Institute of Earth System Sciences

External Organisation(s)

Federal Institute for Geosciences and Natural Resources (BGR)
State Authority for Mining, Energy and Geology (LBEG)

Type

Article

Journal

International Journal of Coal Geology

Volume

295

ISSN

0166-5162

Publication date

04.12.2024

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Fuel Technology, Geology, Economic Geology, Stratigraphy

Sustainable Development Goals

SDG 14 - Life Below Water

Electronic version(s)

https://doi.org/10.1016/j.coal.2024.104645 (Access: Open)