The Position and Age of the Sciotoville Bar Locality, Southern Ohio
The Sciotoville Bar was a rock ledge exposed along the northeast bank of the Ohio River between the Little Scioto River and Wolfred Run (NEJ4, sec. 8, T2N, R20W, Scioto County, Ohio). This well-known fossil locality yielded an abundant fauna from concretions in a shale which formed a portion of the ledge. Since 1920, however, the locality has been flooded by the pool created first from construction of Lock and Dam 31, and later by the Captain Anthony Meldahl Locks and Dam. Information assembled from various sources indicates that the main concretionary horizon represents the upper portion of the Portsmouth Member of the Cuyahoga Formation. In addition, a previously unpublished measured section of this locality by Hyde is now available which describes the location of the concretionary fossil-bearing horizon. The combined ammonoid fauna from the upper Cuyahoga and the overlying lower Logan Formation suggests a Kinderhookian rather than Osagean age for the concretionary horizon.
Mississippian (Lower Carboniferous) miospores from the Cuyahoga and Logan formations of northeastern Ohio, USA
Well-preserved Lower Mississippian (Dinantian) miospore assemblages have been recovered from the upper Cuyahoga Formation (type Wooster Member) and overlying lower Logan Formation (Byer Member) at two localities in Wayne County, northeastern Ohio, USA. All six samples were productive and yielded assemblages that represent the middle Tournaisian Spelaeotriletes pretiosus–Raistrickia clavata (PC) Miospore Biozone of Western Europe including a new species of trilete, acamerate miospore, Mooreisporites streelii, described from the Wooster Shale. Significant numbers of latest Devonian and earlier Carboniferous miospores are reworked into all assemblages. The palynological evidence is consistent with a late Kinderhookian age for most of the Cuyahoga Formation, including its well known ammonoid fauna. However, the position of the boundary between the Kinderhookian and Osagean Series remains uncertain.
Alkali feldspars: ordering rates, phase transformations and behaviour diagrams for igneous rocks
Abstract Homogeneous and heterogeneous phase relationships in the alkali feldspars are reviewed, and behaviour diagrams developed. Al,Si ordering is almost certainly continuous and higher order in both albite and potassium feldspar and has been established reversibly or nearly so down to below 500°C in albite and possibly to ∼ 200°C in potassium feldspar. The degree of order in intermediate albite changes strongly over a range of ∼ 75–150°C depending on pressure, low albite being stable up to about 620–650°C and high albite above about 725°C at low pressure. Symmetry is broken at ∼ 980°C mainly by a cooperative shearing of the whole framework and not by Al,Si ordering alone; there is a thermal crossover near 700°C shearing being dominant above (high albite) and ordering dominant below (intermediate albite). In potassium feldspar symmetry is broken by Al,Si ordering at a temperature of about 500°C The change in degree of order with respect to temperature has been followed easily and reversibly in sanidine from ∼ 1075 to ∼ 550°C and to a lesser extent in microcline from 450 to 200°C. Ordering rates in sanidine down to 500°C and ordering rates in microcline between 450 and 200°C are almost as fast as in albite. Ordering in sanidine at 500°C and below slows and then stops with the development of the tweed orthoclase domain texture. The tweed texture acts as a barrier to further order because the strain energy associated with the (incipient) twin domain texture balances or nearly balances the free energy decrease resulting from ordering. Ordering stops not because of the kinetics of Al,Si diffusion, but because the total driving force is very small or nil. Ordering can readily proceed to completion, with the formation of low microcline, only if the domain-texture barrier is overcome by processes involving fluids or strong external stresses. There is no barrier in albite. The symmetry-breaking process in alkali feldspar changes with composition from mainly shearing in albite to ordering in potassium feldspar. Symmetry is broken equally at a compositional crossover (metastable with respect to exsolution) near Ab80-75 at low pressure and progressively displaced towards Or at higher pressures. Ordering in pure albite occurs by a (nearly) one-step path which progressively becomes two-step with substitution of Or. Diagrams showing the near-equilibrium variation of the order parameters at low pressure with composition and T are given, as well as two extreme phase and behaviour diagrams for complete coherent and complete incoherent (strain-free) relationships. These diagrams can be used to understand feldspar relationships and microtextures in hypersolvus and subsolvus rocks, the occurrence of orthoclase, and of intermediate and low microcline.