Devonian reefs are widespread in Europe, but with the exception of those in Germany and Belgium most have not been widely described in the international geological literature. The present paper aims to amend this situation by providing a general synthesis of published data on all significant European Devonian reef occurrences. Particular attention is paid to their classification and, where possible, to the factors which governed their location and evolution.
The earliest reefs occur in that part of Europe, the “internal” zone of the Hercynian Orogen, which was not strongly affected by the Siluro-Devonian Caledonian earth movements and in which there was continuous marine deposition from the Lower Paleozoic through to at least the Middle Devonian (southeastern Alps, Bohemia, Armorican Massif, and the Cantabrian and Pyreneean Mountains). In the “external” zone (e.g., the “Rhenoher-cynian” be!t, including southwestern England, the Ardennes, Rhenish Schiefergebirge, Harz Mountains, Poland, and Moravia), which experienced uplift and mild deformation on the periphery of the Caledonian Orogen, and where the Devonian usually overlies the Lower Paleozoic unconformably, reef growth did not begin until the Middle Devonian. Here, Lower and early Middle Devonian deposits (and locally the whole Devonian succession) are in a clastic fluviatile or neritic facies. In some cases the Lower Devonian is absent altogether. In most areas, irrespective of this early history, reef growth continued until the late Frasnian or earliest Famennian, when (among other factors) widespread rise in relative sea level caused their extinction, and pelagic sedimentation became the norm.
European Devonian reefs fall into five broad morphological categories: 1. banks, 2. biostromal complexes, 3. barrier-reef complexes, 4. isolated reef complexes (reef-mounds and atolls), and 5. quiet water carbonate buildups (“mud-mounds”). These possess characteristic sedimentary and organic facies associations and distribution patterns. Types 2. to 4. exhibit lateral differentiation into facies-zones (e.g., reef-core and back-reef in atolls, barrier-reef and biostromal complexes, plus fore-reef in most well developed buildups).
Factors which influenced the location and development of reefs in the European Devonian included: 1. local crustal flexures and movement on basin-margin hingelines, 2. synsedimentary faulting, 3. synsedimentary volcanicity, 4. the distribution of small scale elevations on the seafloor (e.g., buried reefs, calcarenite banks), and 5. changes in relative sea level where not clearly attributable to one of the previous factors. Variation in the relationship between reef growth and changes in, or inconsistencies in the rate of change, of relative sea leve! are expressed in basin-marginal reef complexes as sequences of transgression and regression, and in isolated reef complexes as vertical eco!ogical and facial zonation. Successive minor transgressive pulses within broader trends are recorded in the back-reef facies of Middle and Upper Devonian biostromal and barrier-reef complexes of central Europe, and of Middle and Upper Devonian reefs of the southeastern Alps, as depositional cyclicity on the scale of a few meters.
Time-equivalent deposits of European Devonian reefs are shelf and basinal black shales and pelagic limestones. These commonly contain intercalations of reefal debris as turbiditic or “allodapic” limestones.
Following extinction some reefs were buried relatively rapidly by subsequent sediments, whereas others underwent prolonged periods of submarine, or very locally subaerial exposure, and were buried only in the latest Devonian or early Carboniferous. Concomitant with the late growth and early post-growth stages, many reefs developed extensive systems of fissures which were filled both syngenetically and post-genetically with fibrous carbonate cements and internal sediments. The latter commonly contain faunas which are substantially younger than the host reefal limestones themselves.
Early diagenetic carbonate cements are features in many reefal facies, and vary in character from one of these to another. Early vadose cements (microstalactitic and drusy pore linings) are most important in back-reef facies, while in reef-core and fore-reef deposits synsedimentary submarine fibrous cements predominate. In “mud-mounds” stromatactis is an important early diagenetic structure. Dolomitization is extensive in some areas, but is a difficult feature to qualify; some occurrences, especiaUy in central Europe, may be early stage replacement. Later cementation appears to have followed much the same course in all areas for which data presently exist, though neomorphism and response to tectonic stress of the carbonates was more varied.
To date, no significant hydrocarbon deposits have been discovered in European Devonian reefs, although minor traces do occur in central Europe.
Figures & Tables
The voluminous amount of information presented in this Special Publication not only fills a gap in understanding the European approach to reef studies but also provides the necessary data base to allow us (in particular the North American geologist) to incorporate this information in our overall interpretive studies. These studies should serve as an impetus for new investigations and will broaden our understanding of the complex interrelationships that operate in the reef environment.