by Guido Meinhold
Digermulen Peninsula in northern Norway comprises a well-exposed succession of siliciclastic sedimentary rocks which were deposited between circa 630 and 480 Million years ago during Late Precambrian to Ordovician times. Sedimentary rocks are deposited layer by layer on top of each other (Figure 1).
Under ideal conditions there would be a continuous sedimentary record of Digermulen’s geological history. But this is not the case because Digermulen’s sedimentary record is in parts incomplete due to so called “hiatal surfaces” which reflect times of non-sedimentation and/or post-depositional erosion. These stratigraphic gaps can have durations of a few weeks and months or even up to several million years depending if they are of regional or global significance. With this in mind, the geologist knows that portions of the sedimentary archive, meaning parts of the geological rock record, are missing.
Hiatal surfaces can be identified by careful studying the lithology, contact relations and stratigraphic ages between the various sedimentary beds. Good indications for the presence of a hiatal surface are as follows:
1) Abrupt changes in grain size: For example, a mudstone bed is overlain by a conglomerate bed, and the conglomerate contains reworked clasts of the underlying mudstone.
2) Abrupt changes in rock lithology and facies: For example, a limestone bed is overlain by sandstone with, for example, the limestone having been deposited in a marine environment while the sandstone was deposited in a fluvial environment on the continent.
3) Abrupt changes of the geological age of the sedimentary beds: For example, a circa 600 Million years old sedimentary rock is overlain by a circa 580 Millions years old sedimentary rock.
On Digermulen Peninsula, several hiatal surfaces can be studied. One example is related to the so called Mortensnes Formation. This unit comprises mainly silt- to sand-sized material in which up to 30 cm large angular clasts of older (reworked) rock material are embedded (Figure 2). The Mortensnes Formation is a glacially-influenced sedimentary succession deposited during a short time interval in the Late Precambrian. The base of the Mortensnes Formation is represented by a hiatal surface which was formed due to abrupt climatic changes probably circa 580 Millions years ago. A glacial event caused a drop in sea level which led to partial exposure of the land and erosion of the underlying (older) rocks by ice and melt water. The eroded material was redeposited and now forms the main components of the Mortensnes Formation. At the base of the Mortensnes Formation likely a few millions years of Digermulen’s geological rock record are missing.
Another example for hiatal surfaces is found within the so called Manndrapselva Member of the Stáhpogieddi Formation. This unit comprises mainly mudstone, siltstone and sandstone. Occasionally, a few up to 5 cm thick conglomerate layers with well-rounded clasts of up to 1 cm in diameter are found (Figure 3). These layers indicate times of erosion, probably related to tectonic processes. The duration of these stratigraphic gaps is still unknown but they are likely of minor (regional) significance, meaning that within the Manndrapselva Member only a few months or years of Digermulen’s geological rock record are missing.
The formation of hiatal surfaces is often due to prominent changes in the depositional environment. This can be triggered, for example, by climatic changes (e.g., base of Mortensnes Formation) and/or tectonic processes (e.g., likely within the Manndrapselva Member). Hence, recognition of hiatal surfaces is important for better understanding Digermulen’s geological history and for estimation of the rates of abrupt climatic changes and/or tectonic processes.