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Fabric anisotropy induced by primary depositional variations in the silt: clay ratio in two fine-grained slope fan complexes: Texas Gulf Coast and northern North Sea

Lookup NU author(s): Ruarri Day-Stirrat, Professor Andrew Aplin

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Abstract

High-resolution X-ray texture goniometry (HRXTG) has been used to quantify the alignment (anisotropy) of phyllosilicate minerals in a series of diagenetically altered mudstone samples from the deep subsurface of the Texas Gulf Coast and the northern North Sea. The sampled formations have undergone a simple burial history with no overprint of tectonic deformation that may have influenced the phyllosilicate orientation. Observations have been made in two constrained temperature windows 148-150 degrees C (North Sea data) and 169-210 degrees C (Texas Gulf Coast) wherein less than 20% smectite remains in the mixed layer phase illite-smectite. Quantitative X-ray powder diffraction (XRPD) shows quartz (which is dominantly detrital) and phyllosilicate content having an inverse relationship. Quantified illite-smectite fabric intensity (phyllosilicate alignments) ranges from low alignment values of 2.42 m.r.d. (multiples of a random distribution) to high alignment values 6.75 m.r.d. and chlorite + kaolinite ranges from 1.80 to 6.46 m.r.d. We propose that the broad range of phyllosilicate fabric intensity observed reflects the range of quartz/phyllosilicate ratios that are the imprint of depositional setting, bedform truncation, and initial grain size. Detrital quartz acts to disrupt fabric intensity by being non-platy, producing areas sheltered from effective stress ('pressure shadows'), and acting as a matrix support. There is no evidence that bioturbation is a significant control on phyllosilicate alignment in these quartz/phyllosilicate mixtures. These findings have important implications for the role of grain-scale alignment on seismic anisotropy in mudstone systems. Fabric intensity in post-diagenetic samples can be predicted from (1) quartz content and (2) total clay content where: y = 7.4x + 64.8 (1) x is fabric intensity (in m.r.d.) and y is total (wt.%) quartz (2) a = 4.2b + 33.7 and a is total (wt.%) clay content and b is fabric intensity (in m.r.d.). (C) 2010 Elsevier B.V. All rights reserved.

Publication metadata

Author(s): Day-Stirrat RJ, Dutton SP, Milliken KL, Loucks RG, Aplin AC, Hillier S, van der Pluijm BA

Publication type: Article

Publication status: Published

Journal: Sedimentary Geology

Year: 2010

Volume: 226

Issue: 1-4

Pages: 42-53

Print publication date: 03/03/2010

ISSN (print): 0037-0738

ISSN (electronic):

Publisher: Elsevier BV

URL: http://dx.doi.org/10.1016/j.sedgeo.2010.02.007

DOI: 10.1016/j.sedgeo.2010.02.007

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