Facies Characterization and Stratigraphic Evolution of Atypical Upper Maastrichtian Shallow-Marine Deposits of the Reitbrook Formation, North German Basin

The youngest Maastrichtian sedimentary unit in the North German Basin (NGB) is represented by the Reitbrook Formation. Historically significant as a hydrocarbon target, these marine deposits are currently under evaluation for mid-depth geothermal potential. Yet, they remain poorly characterized, with limited understanding of lithofacies variability, depositional environments, and stratigraphic evolution.

This contribution presents preliminary results from a comprehensive sedimentological study of a 140-meter-thick core section of Upper Maastrichtian beds, recovered from the central-northern sector of the NGB in Lower Saxony. Emphasis is placed on lithofacies analysis and the identification of depositional sequences, complemented by existing petrophysical data to assess vertical variations in reservoir quality.

Two lithofacies associations dominate throughout the succession: i) whitish, frequently flint-bearing, sandy marlstones, containing considerable amount of terrigenous component, likely deposited in inner to mid-shelf settings; and ii) glauconite-rich and siliciclastic-dominated “greensand” beds deposited in a nearshore environment, showing high porosity and permeability values. Both associations differ markedly from the classic bioclastic limestones of the Reitbrook Formation described in the literature, have not been previously documented, and are thus interpreted as proximal equivalents.

Sedimentary sequences of various orders are identified within the succession. Particularly, its basal portion comprises stacked small-scale sequences, typically within a 20-meter thickness, characterized by variations in sandy marlstone thickness and flint content, and bounded by glauconite-rich clastic beds. These cycles likely reflect the interplay between sea-level fluctuations and/or episodic pulses of sediment influx. Overall, the succession displays a pronounced regressive trend, with nearshore greensands increasingly dominating upward, correlating with enhanced reservoir potential.