Abstract Experimental compaction curves for mud are key factors in sedimentary basin models but they are poor at predicting porosity evolution beyond 2.5 km depth, where chemical compaction dominates shale diagenesis. This study presents the results of experimental simulation of diagenesis and low-grade metamorphism of an illite shale powder. By applying amphibolite facies conditions using high-pressure–high-temperature deformation apparatus, the transformation of illite to phengite that takes place under anchizone and epizone metamorphic conditions could be accelerated and completed within laboratory timescales. In accordance with observation of naturally compacted shales, our experimentally compacted metapelites range in porosity from 1% to 16% and contain authigenic phengite, biotite, and quartz. By compacting illite shale powder under both lithostatic and non-lithostatic conditions, the role of stress or tectonic forces during diagenesis could be evaluated. Based on microstructural and chemical similarities we infer that both types of samples compact in the same way, indicating that chemical compaction is unaffected by differential stress. Basin evolution models can therefore extend the 1D generalization of the stress field to depths where chemical compaction takes place. However, as chemical compaction enhances mineral alignment, anisotropy of physical properties such as permeability and sonic velocities can change substantially.

ABSTRACT Silurian core samples from the central region of Saudi Arabia provide an excellent insight into the temporal evolution of acritarchs and other related forms. Based on palaeogeographical reconstructions, the area was situated in a temperate region on the eastern margin of Gondwana. The core samples have furnished the basis for site-to-site sequence correlations that made use of biostratigraphy. The composition of the microflora changes rapidly and distinctly, and the palynological composition of acritarchs and related forms reflect the nature of changes in the depositional environments immediately above the uppermost Ordovician. The uppermost Ordovician and lowermost Silurian interval is characterised by a low diversity of simple forms of acritarchs and some prasinophytes. The acritarchs recovery is marked by the appearance of innovative morphologies in the atavus-acinaces Zones above the “hot shales” interval, and distinct microfloral turnovers occurred later. Some similarities are evident between the Rhuddanian assemblages of Saudi Arabia and those of North America. By comparison with the acritarch biozonation established in the type areas of Great Britain, the precocious appearance of taxa such as Carminella, Leprotolypa gordonense, Visbysphaera , and others, occurs in this part of the northern Gondwanan margin. Characteristic Telychian species allow precise correlations with the Silurian sequences of southwestern Libya and Parnaiba Basin of Brazil. A readjustment of the age of the previously defined acritarch Biozones 6 and 7 (Le Hérissé et al., 1995) is proposed taking account of the details and stratigraphic precision revealed here by the study of additional samples.

Abstract The Red Mountain Formation is an unconformity-bounded unit including all Silurian strata in the Appalachian Valley and Ridge province of Alabama and Georgia. It is also host to the well-known Birmingham iron-ore field. The formation is divided by paraconformities into six members that generally coincide with depositional sequences. From biostratigraphy, the lower four members are established as Llandoverian: Taylor Ridge (Rhuddanian), Duck Springs (? early Aeronian), Birmingham (middle Aeronian-early Telychian), and Ruffner (late Telychian). Upper members are: Rocky Row (Wenlockian) and Sparks Gap (Pridolian). Facies indicate deposition on a storm-dominated shelf with coarse-grained, cross-bedded sandstone in the shoreface passing seaward into hummocky cross-bedded sandstone and shale on the inner shelf, and interbedded shale and graded sandstone or limestone (storm beds) on the outer shelf. Unless truncated by erosion, all Llandoverian sequences consist of thin retrogradational facies successions in transgressive systems tracts and thick progradational successions in highstand systems tracts. Accommodation on the shelf was provided by a combination of flexural subsidence, driven by tectonic loading of the Appalachian orogen during waning stages of the Taconic orogeny and glacial eustasy. It is possible to recognize similar lithofacies and sequences at least as far north as New York State. Coarse-grained, ferruginous, cross-bedded sandstones (ironstones) occur as sharp-based shoreface facies associated with sequence boundaries; at the base of the transgressive systems tract, or making up the lowstand systems tract or falling stage systems tract. Such shoreface deposits are commonly highly condensed with ooids of hematite-chamosite and skeletal debris coated and replaced by hematite. Mineralization was evidently favored by periods of sediment starvation and reworking on wave (transgressive systems tract) and tidal (lowstand systems tract) ravinement surfaces. However, the richest ores in the Birmingham district (Big and Irondale seams) developed in a falling stage systems tract apparently as a consequence of meteoric diagenesis during forced regression.