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Title: Sequence Stratigraphy of Late Namurian (Marsdenian to Yeadonian) delta systems in northern England
Author: Church, Kevin Donald
Awarding Body: The University of Manchester
Current Institution: University of Manchester
Date of Award: 1994
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Abstract:
This study is the first regional synthesis and sequence stratigraphic interpretation of late Namurian delta systems in northern England to use subsurface data. The studied interval provides excellent regional coverage, both in terms of outcrop in the Pennines, and a detailed subsurface database produced during oil and coal exploration over the past 76 years, principally in the East Midlands and environs. Lithofacies analysis of outcrop sections, aided by gamma ray spectrometry, has enabled the interpretation of log facies from wireline logs and core data in the subsurface. Facies stacking patterns have been used to identify retrograding, aggrading and prograding parasequence. sets, key stratal surfaces and systems tracts. The correlation of well and outcrop sections is well constrained within a rigid biostratigraphic framework, defined by goniatite fauna within 'marine band' mudstones, which interfinger between each phase of delta progradation. Across the study area, up to 10 high frequency (fourth-order) depositional sequences are recognised within the Marsdenian-Yeadonian (late Namurian). Thick sandbodies have traditionally been assigned to a delta top, distributary channel environment. Although this may be true for some sandbodies (e. g. the Chatsworth Grit of the Widmerpool Gulf), others are interpreted to have formed during subaerial exposure and incision of the underlying highstand delta system, during relative sea-level fall (e. g. the Ashover Grit and Rough Rock of this study). The cutting out of marine bands by these incision events has important implications for the potential miscorrelation of these vital stratigraphic markers over uncored intervals and for the subsequent miscorrelation of sandbodies. The small thicknesses exhibited by transgressive systems tracts suggests that relative sealevel rises were able to markedly outpace sediment supply and were therefore probably rapid. Falling stage systems tract deposits are also likely to make up a significant component of some sequences, though difficulties in recognising the sequence boundary in well data hinders the separation of late highstand from falling stage systems tract progradation. Sequence thicknesses show systematic variation in the late Namurian, allowing the preferential development of particular systems tracts and preservation of particular stratal surfaces. Such sequences are grouped into sequence sets which were controlled by a longer period (third-order) cycle of relative sea-level rise and fall during the Marsdenian-Yeadonian. Additionally, these sequence set stacking patterns control the distribution of reservoir sandstones. In particular, incised valley fills, by cutting out permeability barriers and connecting sandstones of successive delta systems, will complicate the hydrocarbon play fairway. The juxtaposition of lowstand valley fill sandstones against highstand delta top sandstones also has important implications for reservoir geometry, continuity and heterogeneity. Palaeogeographical maps for selected systems tracts have been produced. These show that although the distribution and orientation of the major fluvial channels was initially influenced by differential compaction over underlying Dinantian syn-rift structures, this influence decreased during the late Namurian. Major fluvial channels, in addition to defining the distribution of delta top conditions, shoreline positioning and systems tract thickness, were instrumental in controlling sequence and sequence set architecture in the late Namurian. Laterally within each sequence, delta thicknesses and the degree of shoreline progradation may vary widely. The sequence stacking patterns observed are explicable in terms of relative sea-level fluctuation, though sequence stacking patterns were heavily influenced by variations in the sediment transport pathways. In some cases, sediment flux was so important that retrogradation in one area was contemporaneous with progradation in another. Quantification of sediment volumes, local basin subsidence and sediment compaction, and their application to the derived relative sea-level curve is clearly the way ahead if a better understanding of the controls on delta development is to be achieved
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.504508  DOI: Not available
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