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Geochemistry of Devonian Reefal Limestone of the Klutert Cave, Germany

Master's Thesis 2017 30 Pages

Geography / Earth Science - Geology, Mineralogy, Soil Science

Excerpt

Table of Content

1 Abstract

2 Introduction
2.1 Motivation:
2.2 Reef Builder of the Klutert Mountain
2.3 Classification of reefs

3 Geological Setting of the Klutert Cave
3.1 Locality and Geology of the Klutert Cave
3.2 Paleogeography

4 Material &Methods
4.1 Component Mapping
4.2 LiDAR Measurement
4.3 Cathodoluminescence
4.4 Electron Microprobe Analysis (EMPA)
4.5 Gas source mass spectrometry

5 Results
5.1 Component Mapping
5.2 LiDAR
5.3 Cathodoluminescence
5.4 Element Mapping
5.5 Gas source mass spectrometry

6 Discussion
6.1 Component Mapping
6.2 LiDAR
6.3 Cathodoluminescence
6.4 Element Mapping
6.5 Gas Source Mass Spectrometry

7 Conclusion

Acknowledgement

References

1 Abstract

The lower coral limestone of the upper Honsel beds is a transition zone between the argillaceous-dominated lower Honsel Beds and the reefal Massenkalk. The Klutert Cave in Ennepetal was formed by dissolution of carbonate from the lower coral limestone, which is Devonian in age. Component mapping of the cave wall shows a decrease in abundance and size of domical stromatoporoids and massive corals towards the top of the reef emphasizing a worsening of living conditions due to increasing sediment influx. LiDAR scans give no additional information due to the rough cave surface. Cathodoluminescence shows at least 2 marine cement phases and 4 cement phases in burial realm. Meteoric dissolution and precipitation is obvious on large scale but absent on sample scale. Element maps show an increasing of diagenetic alteration in skeletons and cement close to late diagenetic fractures due to low permeability of the matrix. Oxygen and carbon isotope data show a bimodal distribution similar to Devonian marine and burial cements from the Golden Spike Reef, Alberta, Canada.

2 Introduction

2.1 Motivation:

Carbonates hold about half of the world´s oil and gas and large amounts of mineral resources, so they are of great economic importance (Flügel, 2009). Despite that, rather little is known about carbonate reservoirs. Most of the data come from well cores, drill cuttings and geophysical measurements (Tucker & Wright, 1990). The Klutert cave is the unique situation to study a reef being able to move within the reef body. Making use of the detailed surface of the Klutert Cave, the thesis addresses the following three questions: 1. In what kind of environment did the reef form? What type of reef building organisms did form the reef? 2. What kind of diagenetic realms did the skeletons pass through? 3. Does alteration affect skeletons to a varying extent according to the organism? The knowledge gained from this study could be applicable on carbonate reservoirs which are not accessible to direct observation and sampling.

2.2 Reef Builder of the Klutert Mountain

Stromatoporoids: Stromatoporoids are extinct calcareous, sessile fossils. Stromatoporoids were placed into the Hydrozoa but are now recognized as a separate class of Porifera. They are present in the geological record of the Paleozoic and Mesozoic. The diameters vary between few cm to m. Fossils exist in various shapes (Flügel, 2009). Aktinostroma present in the study area shows tabular and domical shape.

Corals: Tabulate and rugose corals are common in Paleozoic shelf and platform environments. They belong to the class of Anthozoa. Tabulate corals exist from early Ordovician to late Permian. They are colonial and form colonies up to several m. Laminar tabulate corals overgrow the stromatoporoids and branching tabulate corals baffle sediment in the study area. Rugose coral can be solitary or colonial. They range from Ordovician to late Permian (Flügel, 2009).

2.3 Classification of reefs

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Figure 1: classification of reefs according to the main supporting component (Riding, 2002)

Classification of marine biogenic structures is a topic highly discussed among geologists. In this thesis, the term reef is used in a broad sense using the definition of Riding (2002). Reefs are defined here as all calcareous deposits created by essentially in place sessile organisms. In order to distinguish reef categories, the internal structure is used. The internal structure derives from the shape and the arrangement of the supporting components as seen in figure 1. The three components in this classification system are the matrix resulting from sediment trapping, the syn-sedimentary cement and the skeleton deriving from the formation of skeleton.

3 Geological Setting of the Klutert Cave

3.1 Locality and Geology of the Klutert Cave

The Klutert cave is located at the base of the Klutert Mountain, which borders with Ennepetal to the south. Ennepetal is a city on the north-west of the Sauerland, about 30 km south of Bochum. This area belongs to the northern edge of the Rhenish Massif in the center of North-Rhine-Westphalia (Koch, 1992). The Klutert Mountain consists of lithified marine sediments of upper middle Devonian age. The Red shale is the oldest member forming the Klutert Mountain. The argillaceous material is deposited as the uppermost part of the lower Honsel Beds. It consists of weathering debris from the Old Red Continent and forms the ground of the Klutert cave in some areas (Koch, 1992).

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Figure 2: Members of the Klutert Mountain (Koch, 1992)

The “greywacke layer” is the lowest layer of the upper Honsel Beds and consists of dark-gray,marine sandstones. It has a maximum thickness of 2 m and exists only in parts of the cave (Koch, 1992). The lower coral limestone (UntererKorallenkalk/ UntereKalksteinabfolge) starts with a 50 cm thick layer of limestone of reef building organisms. This layer is covered by a 1 m thick layer of marly sandstone. Above the sandstone, the actual lower coral limestone appears. It consists of stromatoporoid and coral-skeleton rich limestone in a crinoid and brachiopod rich matrix. This layer has a thickness of 2 – 12 m and a lateral extent of 400 m (NS) and 200 m (EW) exposed in the Klutert Cave. The lower coral limestone dips to the north by 10°. This layer was formed by isolated plateau reefs that covered a surface of at least 2000 * 1500 m (Koch, 1992). It separated due to tectonic stress. The Klutert cave was formed by the dissolution of the carbonate rocks of this layer.

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Figure 3: Lithological members of the study area in the Klutert Cave.

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Figure 4: Geology of the northern Sauerland (after Basse et al., 2016)

The Formation of the cave started in the Paleogen and is still active in the northern part. The Hanging wall is the shale interlayer (untereSiltsteinfolge/ Schieferzwischenmittel) with a thickness of about 50 m. It consists of sandstone, sandy claystone and siltstone with fossil rich horizons that cannot be laterally correlated (Koch, 1992).

3.2 Paleogeography

The studied rocks of the lower coral limestone formed about 380 Ma during the middle Devonian (Koch, 1992). At that time the Old-Red Continent and the Gondwana Supercontinent were separated by the Rheic Ocean. The study area was located on the south-east shelf of the Old-Red continent between 10° and 20°S, thus in the tropical to subtropical climate zone (fig. 5).

In general, the sea-level rose during the Devonian with temporary deviations (Koch, 1992). So the coast line moved gradually to the north and north-east. Terrigenous sandstone, siltstone and claystone were deposited in river deltas during the Eifelian and early Givetian on the north-west Sauerland (May & Marks, 2013). Similar sedimentation is found on the bordering Ennepetal area. The upper Honsel Beds consist of silt- and claystone with low sand content and interstratified coral limestones. Due to that, the upper Honsel Beds are considered as a transition phase between the more proximal and sand-rich lower Honsel Beds and the more distal reefal limestones of the Massenkalk (May & Marks, 2013).

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Figure 5: Paleogeography of land masses in the middle Devonian (after Basse et al., 2016). Position and distance between Laurussia and Gondwana is under constant discussion.

The sediments of the study area were deposited under shallow-marine conditions. The shelf was subdivided into troughs, ridges and islands (Polenz, 2008). Large amounts of terrigenous sediments were brought into the marine troughs (Koch, 1992). This sediment influx varied according to the relative sea level (Polenz, 2008). In times of low terrigenous sediment influx, isolated patch reefs formed on the ridges (Polenz, 2008; Koch, 1992). These reefs were of limited thickness and lateral extension (Koch, 1992). When the sediment influx eventually increased, these reefs were covered and died (Basse et. al., 2016).

4 Material &Methods

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4.1 Component Mapping

A wall inside the cave was chosen as study area. The area is 3.5 m high and 1 m wide. A grid of 25 cm sized squares subdivides the area into 48 units. Each unit was mapped according to the biogenic and abiogenic components as they are visible. Biogenic components include rugose corals, Favosites, Thamnopora and other tabulate corals, stromtoporids and brachiopod tempestite. Furthermore, stromatoporoids were classified according to Kershaw (1998) into tabular and domical forms.

4.2 LiDAR Measurement

In addition to the visual mapping a LiDAR (Light detection and range) measurement was conducted on the study area and the surrounding area to map it according to the area´s clay and carbonate content. The study area was scanned with a Faro 3DX_330. The received data were processed with Faro Scene and CloudCompare.

A LiDAR is a system similar to radar, emitting light waves instead of radio waves (Young, 2011). A laser beam is directed towards the wall. The Intensity of the reflection depends on the chemical composition. Clay-rich components absorb more of the emitted light than carbonate components do. Therefore, carbonate rich fossils show high intensity and clay rich components show low intensity (Mathias Knaak, pers. comm.).

The intensity of the reflection is not only depended of the chemical composition but also of the distance. The initial beam diameter is 2.2 mm. The diameter increases with travelling distance. The intensity of the reflected signal depends on the mean distance between the scanner and the surface within the lighted spot (Mathias Knaak, pers. comm.). Surfaces normal to the beam can be mapped in 3D with high precision. Surfaces with high relief are difficult to map (Young, 2011). Due to the high relief, the influence of the distance on the intensity overlies the influence of chemistry. Also, edge effects occur on the map.

4.3 Cathodoluminescence

Rocks were collected from the cave and 9 thin sections were prepared of these samples. These thin sections have a thickness of about 30 µm. After that the thin sections were polished from both sides and were not covered. A thin layer of gold was attached on the sample in order to avoid electrical charging during measurement. The measurement was conducted with a Lumix HC1LM cathodoluminescence microscope with a hot cathode. Mn2+ is the main activator for cathodoluminescence. Also, Pb and rare earth elements act as activators. Fe2+ acts as inhibitor. Above 2 wt% FeCO3 no luminescence is shown. Lumination is more depending on the ratio of Mn2+ and Fe2+ than the actual concentration (Tucker & Wright, 1990).

4.4 Electron Microprobe Analysis (EMPA)

On 4 of the thin sections the gold layer was removed. These were analyzed with a Microprobe. The electron microprobe analysis is a technique for analyzing solid samples like thin sections of rocks, according to their elemental composition. In geology, this technique can be used to identify minerals and their diagenetic features. During the process, a focused electron beam is directed to the sample that causes radiation of X-rays. The detection limit is about tens of parts per million (ppm) and spatial resolution is limited to about 1 μm. Element composition can be recorded on certain points of interest, in the form of line profiles or two-dimensional maps.

The electrons of this beam are decelerated and deflected by interacting with the outer electrons or the atomic nuclei from minerals in the sample. In both cases, some of these electrons can leave the sample again and are referred to as secondary electrons and back-scatter electrons (BSE) both of which are used for imaging purposes. The amount of back-scattered electrons is determined by the mean atomic number of the sample material. Therefore, in the BSE images bright regions correspond to heavier elements, whereas BSE-dark regions contain comparatively lighter elements.

Electrons that interact with the Coulomb field around the atomic nucleus cause the emission of X-ray photons in a continuous X-ray spectrum. When electrons pass the inner atomic shells causing the ejection of an electron from that shell, the vacancy is filled by an electron from an outer orbital. Electrons changing to an inner orbital give rise to characteristic X-rays. Due to the specific electron configuration of different elements, the wavelength and the energy of these X-rays are characteristic for the elements that caused their emission. By measuring the X-ray intensity, the elemental concentration in the sample can be deduced. In this study, measurements are conducted with a primary beam acceleration voltage of 15 kV. Ka lines are used for the elements aluminum, calcium, iron, potassium, magnesium, manganese and silicon. For strontium, the Ma lines are used.

The instrumentation of an electron microprobe is similar to that of a scanning electron microscope (SEM). It contains an electron gun that accelerates electrons towards the sample. This electron gun is placed on top of the instrument. The electron beam is focused on the sample by electron optical lenses. An optical microscope is mounted coaxial to the electron beam. The instrument must be evacuated to allow the electrons to reach the sample as well as to protect the electron source. Operating pressure should be below 10-5 mbar. An EMPA differs from a SEM by the detectors used. The EMPA has two types of X-ray spectrometers. The energy-dispersive (ED) type can record X-rays of all energies at the same time. This type is commonly used to measure up to ten elements and to plot intensity versus X-ray photon energy. The wavelength-dispersive (WD) types can measure only one element, each using the Bragg reflection by a crystal. This is less convenient but gives better spectral resolution and peak-to-background ratio (Niels Jöns, pers. comm. and Reed, 2005).

For this study, the elements iron (Fe), strontium (Sr), manganese (Mn) and magnesium (Mg) were measured with the WD. Aluminum (Al), potassium (K), phosphor (P) and silicon (Si) were measured with ED. Calcium (Ca) was measured with both types. The thin section is mapped by moving the sample under the electron beam. Every pixel of the map equals one measurement. When scanning entire thin sections, resolution is limited in order to keep operation time reasonable.

4.5 Gas source mass spectrometry

10 samples have been taken from a carbonate rock in order to measure their δ13C and δ18O. The isotopes have the same number of protons and electrons but different numbers of neutrons. The isotopes of the same element have similar chemical properties. Their physical properties differ from each other due to their different masses, resulting in isotopic fractionation during biogenic and physical processes. These changes in isotopic composition are usually very small compared to the ratio of isotopes. This is why relative measurements are carried out. For this study the ratio of the rare to the abundant isotope was measured.

0.10 mg to 0.11 mg per sample has to be weight in and filled into headspace vials. Before measurement, the samples were dried in the oven at 105° C for at least 24 hours. Afterwards the vials have to be flushed by helium to remove the air. For the gas source mass spectrometry measurement, carbonate samples have to be converted to gas. Therefore, the sample is converted to CO2 by reacting with concentrated phosphoric acid (H3PO4). A double hollow needle streams helium through a septum on top of each vial. The He-sample gas mixture passes a semipermeable polymeric membrane, where water is removed from the gas.

The gas phase is ionized by a bombardment of electrons, which are emitted from a heated rhenium filament. After that the ions are accelerated by an electrostatic potential in a high voltage field of 10 kV. Slits in charged plates focus the beam towards a magnetic field, in which ions are deflected according to their mass/charge ratio. Lighter isotopes are deflected stronger than heavier isotopes. Ions of each isotope are counted in one faraday cup. Isotope ratio can be calculated by the signal ratio between isotopes and between sample and standard (Breitenbach, 2011 and Niedermeyr, 2016).Knowing the initial marine δ18O and the δ18O of the calcite the crystallization temperature can be estimated. A negative shift of δ18O by 2 ‰ correlates with an increase of 10 °C. This calculation allows only a rough estimate since the composition of pore fluid is influenced by many factors (Kaufmann, 1997, after Friedman and O´Neil, 1977).

5 Results

5.1 Component Mapping

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Figure 6: The study area (6a) mapped according to the biogenic and abiogenic components, (6b) as merged photo and (6c) as LiDAR scan.

The study area shows stromatoporoids and corals of various sizes and shapes. Tabulate corals form large colonies. Rugose corals are rare and form small solitary organisms. Stromatoporoids are mainly in growth position and rarely in situ.

0-1 m: Large skeletons of tabular and domical stromatoporoids and thick, tabulate corals growing on the stromatoporoids are dominant. Skeletons are in contact to each other and form a complex pattern. In this section stromatoporoid skeletons are the main volume contributors with about 50 % of the surface covered by these organisms. About 15 % of the bottom section area is covered by massive tabulate corals. At 0.5 m, branching corals occur for the first time, forming branches of cm scale surrounded by brown to gray matrix. They cover about 5 % of the area in this section. There are neither signs of boring, nor of encrusting on any kind of organism. Carbonate skeletons show fractures healed with light gray calcite.

1-2 m: At about 1 m stromatoporoids are no longer in contact with each other. Tabulate corals grow on stromatoporoid skeletons. The thickness of these corals is smaller than in the bottom section, usually does not exceed 10 cm. Stromatoporoids are of limited size and show tabular shape. Most of them are horizontal, few are vertical. Branching corals are the most abundant component in this section, with large amount of sediment baffled between skeletons. About 50 % of the section area is covered by this unit. Stromatoporoids cover about 20 %, massive tabulate corals below 5 % of the area. The matrix without corals is limited to areas where tabular stromatoporoids grow very close to each other.

2-3.1 m: Here the size of stromatoporoids increases. Skeletons show domical and tabular shapes. The thickness of tabulate corals overgrowing stromatoporoids remains limited to several cm. Bulbous Favosites are common.

3.1-3.5 m: Dark gray shale covers the lower coral limestone. In the study area the shale is interstratified with brachiopod tempestite. These layers are up to 10 cm thick. Above the study area no tempestite layers are exposed.

5.2 LiDAR

The LiDAR scan shows dark, clay-rich areas mostly at the upper part of the study area. In some areas round, light colored areas contrast from the dark gray surrounding (fig. 6.1). They have a size of up to 20 cm. Above these areas there is a black irregular shaped area in the scan. At the very top of the study gray area layers contrast from the surrounding darker areas (fig. 6.2).Towards the bottom the scan is vaguer (fig.6.3). Very bright, horizontal lines are wider to the bottom. Besides that significant contrasts are not observed.

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Details

Pages
30
Year
2017
ISBN (eBook)
9783668451513
ISBN (Book)
9783668451520
File size
9.7 MB
Language
English
Catalog Number
v365460
Institution / College
Ruhr-University of Bochum
Grade
Tags
Ennepetal Klutert Höhle Honseler Schicht Korallenkalk

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Title: Geochemistry of Devonian Reefal Limestone of the Klutert Cave, Germany