Abstract

In the Lower Rhine Basin, a block-faulted subsiding area since the early Tertiary, thick lignite seams developed in the setting of continuous competition between the terrestrial and marine realms. In the western part of the basin the ~ 2.5 km long by ~ 175 m deep lignite opencast mine “Zukunft-W” (Eschweiler, West Germany) was studied. The shifting exposures permitted a spatial facies analysis of the suite of siliciclastics accompanying and overlying the upper Miocene Main and Upper Seam groups (Hauptfloz und Oberfloz Gruppe). The roofing sediments are of Pliocene and Pleistocene age.

Field data were combined into a single detailed north-south section, 2 km long and 170 m high (Fig. 1). Special attention was given to the arrangement of lithofacies in vertical and lateral sequences of deposition. The following lithofacies (assemblages) were distinguished (in order of increasing terrestrial influence):

1. Transgressive marine sandsheet

2. Flood-tidal delta with inlet and spillover subfacies

3. Washover sand

4. Blowover (eolian) sand

5. Open lagoon

6. Backbarrier lagoon

7. Bayfloor

8. Bay margin

9. Swamp feeder channel

10. Minor distributary channel

11. Major distributary channel; fine grained, meandering

12. Crevasse splay

13. Major alluvial river; coarse grained, meandering

14. Braided river

15. Streamflood system.

These facies combine to represent six successive sedimentary episodes:

a. The marine onset

b. The lower deltaic plain

c. The marine onlap

d. The upper deltaic to alluvial plain

e. The marginal marine incursion

f. The final predominance of the alluvial plain.

The intercalated lignite layers have quite variable thickness, lateral continuity and levels of impurity. There appears to be a correlation between these characteristics of the seams and their environmental setting. Thick, continuous and relatively uncontaminated seams developed preferentially in the (back) barrier setting. The deltaic plain gave rise to rather irregular peat growth: layers tend to split up and unite, but are usually of small to intermediate thicknesses. The admixture of siliciclastics is also variable, but usually restricted to the fine grades. Lignite layers in the alluvial plain setting tend to be thin, discontinuous and often laterally replaced by silts and clays. Sedimentation responded to regional and local subsidence, compaction and eustatic changes in sea level. The effects of local, fault-controlled, higher subsidence rates and a eustatic rise in sea level are represented by streamflood and marine onlap units respectively.