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

摘要

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 ℃ (North Sea data) and 169-210 ℃ (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:rny = -7.4x + 64.8 (1)rnx is fabric intensity (in m.r.d.) and y is total (wt.%) quartzrna = 4.2b + 33.7 (2)rnand a is total (wt.%) clay content and b is fabric intensity (in m.r.d.).