Petrology and geochemistry of the carbonates, Ballagan Formation, N.W. Midland Valley, Scotland

Ghummed, Milad Ali (1982) Petrology and geochemistry of the carbonates, Ballagan Formation, N.W. Midland Valley, Scotland. PhD thesis, University of Glasgow.

Abstract

This study investigates the nodular and stratified carbonate
beds 1n the Ballagan Formation, in the Western Midland Valley of
Scotland. The Ballagan Formation, which also includes lutltes and
quartz arenites, lies stratigraphically between the Upper Old Red
Sandstone end the Spout of Ballagan Sandstone: it constitutes the
lower-most part of the Calciferous Sandstone Measures.
Microscopic examination of thin sections showed that the
carbonates comprise mainly three microfacies which are subdivided on
the basis of fabric and crystal-size. Microfacies A is the
finest-grained and from it the other two have diagenetically
evolved; through neomorphism (Microfacies B), and metasomatism and
segregation (Microfacies C). Microfacies B has resulted from
multiple neomorphic stages as indicated by crystal-size variation.
As a, .result of neomorphism, clay has concentrated In .the intercrystalline
boundaries, leaving the new crystals slightly clearer
than their precursors. Microfacies C has developed in two ways:
(1) metasomatism and (2) segregation. Calcitization of dolomitic
beds and segregatIon of calcite in an original argillaceous
sediment, both produced Microfacies C. The controlling factors
over these processes are unknown.
Shrinkage cracks, cavity-cement, and veinlets are common
features in both stratified and nodular carbonates. Poorly
preserved laminations are uncommon in the untreated rock specimen,
but are common in thin section. Whilst it is difficult to prove
an algal origin for these structures, they morphologically
resemble algal laminations.
Calculation of mineral proportions from chemical analyses by
X-ray fluorescence show that c 86% of the carbonate beds contain
more than 50% of the mineral dolomite, therefore, they are generally
dolomites by definition, with minor limestone occurrences.
Terrigeneous material content is composed mainly of clay minerals;
illite, chlorite, and montmorillonite with common quartz. Gypsum is
a minor lithology in the rock assemblages.
Electron microprobe analysis has shown that crystals of both
Microfacies A and B are composed mainly of dolomite, the crystals of
the first contain more clay than those of the latter. Crystals of
Microfacies C are composed of calcite. Probing of veinlets confirmed
a wide range of mineral compositions.
From a consideration of the fineness, bed-thickness, structures,
faunas, composition, rock-association and lateral facies relationships,
these beds are thought to have formed in a lagoonal environment.
On the seaward the lagoon was probably bounded by sand bars;
on the landward by caliche pavements and alluvium. The best
analogous environment is seen in the Coorong, 5. Australia, where
fine dolomitic beds are laid down during the wet season and
desiccated during the dry.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QE Geology
• Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences
• Supervisor's Name: Bluck, Dr. B.J.
• Date of Award: 1982
• Depositing User: Ms Dawn Pike
• Unique ID: glathesis:1982-4870
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 23 Jan 2014 12:03
• Last Modified: 23 Jan 2014 12:03
• URI: https://theses.gla.ac.uk/id/eprint/4870