Barrie Rickards

Transpressive duplex and flower structure: Dent Fault System, NW England

Abstract Revised mapping along the Dent Fault (northwest England) has improved the resolution of folds and faults formed during Variscan (late Carboniferous) sinistral transpression. A NNE-trending east-down monocline, comprising the Fell End Syncline and Taythes Anticline, was forced in Carboniferous cover above a reactivated precursor to the Dent Fault within the Lower Palaeozoic basement. The Taythes Anticline is periclinal due to interference with earlier Acadian folds. The steep limb of the monocline was eventually cut by the west-dipping Dent Fault. The hangingwall of the Dent Fault was dissected by sub-vertical or east dipping faults, together forming a positive flower structure in cross-section and a contractional duplex in plan view. The footwall to the Dent Fault preserves evidence of mostly dip-slip displacements, whereas strike-slip was preferentially partitioned into the hangingwall faults. This pattern of displacement partitioning may be typical of transpressive structures in general. The faults of the Taythes duplex formed in a restraining overlap zone between the Dent Fault and the Rawthey Fault to the west. The orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly. Kinematic constraints provided by the Dent and neighbouring Variscan faults yield a NNW–SSE regional shortening direction in this part of the Variscan foreland.

Upper Ordovician chitinozoan biostratigraphy from the type Ashgill area (Cautley district) and the Pus Gill section (Dufton district, Cross Fell Inlier), Cumbria, Northern England

Seventy-five samples from the classic sections through the historical type area of the Ashgill Series in the Cautley district and along Pus Gill in the Cross Fell Inlier have been examined for chitinozoans. The results of this study allowed the recognition of five internationally recognized biozones and the definition of two new Avalonian chitinozoan zones. From bottom upwards, these are: the Fungochitina spinifera , the Tanuchitina bergstroemi ?, the Conochitina rugata , the Spinachitina fossensis , the Bursachitina umbilicata sp. n., the Ancyrochitina merga and the Belonechitina postrobusta zones. One new species is described: Bursachitina umbilicata sp. n. This biozonation enables a correlation between the Cautley district and the Baltoscandia and Gondwana palaeocontinents based on chitinozoans. The Baltoscandic chitinozoan zones are, therefore, now better correlated with the British chronostratigraphical scheme, which is still widely used. It is stratigraphically significant that the base of the Ashgill in its type area does not fall within the Tanuchitina bergstroemi Zone, as widely believed before, but in the Fungochitina spinifera Zone. In addition, chitinozoans from the Onnian (Caradoc) section of the Cross Fell Inlier provide a link with the type Caradoc section in Shropshire.

Ordovician and Silurian graptolite assemblages from Cerro del Fuerte, San Juan Province, Argentina

A description is given of three transects through the Silurian strata of Cerro del Fuerte, San Juan Province, Argentina. All are in the Precordillera structural belt; the Silurian strata probably have conformable boundaries with the Ordovician and Devonian, and there is a regional angular unconformity at the base of the Carboniferous. The Silurian includes two formations, the La Chilca below and the Los Espejos above. Both formations exhibit a coarsening upwards sequence: the Los Espejos Formation spans the Wenlock, Ludlow and, probably, Přidoli epochs, the first being of relatively fine sediments and the latter two of coarser sediments. The third transect also includes Devonian sediments which seem to represent, albeit incompletely, a third coarsening upwards cycle. The graptolite faunas have been extensively collected and form the basis of the systematic section of the paper. Late Ordovician, Llandovery and Ludlow faunas are described.

Psigraptus jacksoni Rickards & Stait―systematics, reconstruction, distribution and preservation

Species of Muenzhigraptus, Diphygraptus, Hunjiangoraptus (sic), Holopsigraptus and Neoclonograptus from China are placed in synonymy with Psigraptus jacksoni Rickards & Stait. Extensive collections of P. jacksoni from the Florentine Valley Formation show the fully grown colony to have been a reclined, ‘shrubby’ cone with at least thirty terminal stipes. The species is indicative of mid-Tremadoc rocks. The preservation suggests that the stolons may have been only partially sclerotised. Three forms of preservation are present within the one bed, only one form of which shows the zooids and stolons and then very rarely. Local variations in microenvironments on and beneath the sea floor are postulated as the cause of variation in form of preservation.

Clingfilm preservation of spiraliform graptolites: Evidence of organically sealed Silurian seafloors

The Early Silurian graptolites Spirograptus turriculatus and Sp. guerichi are typically preserved in condensed graptolite shale lithologies as entirely flattened outlines; all whorls are visible and little or no sediment infills the originally cone-shaped rhabdosomes, as though the graptolites had been sealed in clingfilm, or plastic wrap, prior to burial and compaction. By contrast, the rhabdosomes of graptolites transported in turbidity currents typically are filled with sediment. The most likely reason for the clingfilm mode of preservation is encasement or covering of the graptolite rhabdosomes by marine snow and/or microbial mats prior to burial by clastic sediment and compaction on an anoxic seafloor. Experimental evidence reported herein supports this suggestion. The organic material that mediated the preservation of such graptolites was likely akin to, but probably physically stronger than, the delicate benthic flocculation layer of the current Black Sea floor. Like the latter, it probably formed significant microtopography at and just below the sediment- water interface and mediated geologic processes such as early diagenesis in graptolite shale lithologies.

Late Silurian (Pridoli) graptolites from the Wallace Shale, New South Wales

A graptolite fauna from the Wallace Shale at Cheesemans Creek, west of Orange, central west New South Wales, gives a clear Pridoli age for the lower half of the Wallace Shale in that area. Previous identifications of species from the Wallace Shale are revised in a description of the fauna. The following taxa are described: Dictyonema delicatulum cf. barnbyensis, Pristiograptus shearsbyi, Monograptus transgrediens, M. prognatus, M. bouceki and M. hornyi. Three evolutionary lines are suggested for the M. bouceki species group: a) M. uncinatus praeprognatus M. prognatus → M. uniformis; b) M. uncinatus s.l. → M. pridoliensis; c) M. uncinatus s.l. → M. hornyi → M. bouceki.

Enigmagraptus n. gen., a new graptoloid (Přídolí, Silurian, New South Wales, Australia)

Enigmagraptus n. gen. comprises a group of the smallest known graptolites, with problematical evolutionary ancestors. The species described, all of Přídolí age are:E. yassensis (Rickards &Wright, 1999),E. cf.yassensis, E. mitchelli (Rickards &Wright, 1999) andE. pennyae n. sp.KurzfassungEnigmagraptus n. gen. umfasst eine Gruppe der kleinsten bislang bekannten Graptolithen mit unbekannten Vorfahren. Die beschriebenen ArtenE. yassensis (Rickards &Wright, 1999),E. cf.yassensis, E. mitchelli (Rickards &Wright, 1999) undE. pennyae n. sp. stammen alle aus dem Přídolíum.