James A. Dockal

Thermodynamic and kinetic description of dolomitization of calcite and calcitization of dolomite (dedolomitization)

Replacement dolomitization of calcite is a chemical process where dolomite comes to occupy the spatial position and volume of calcite through simultaneous dissolution of calcite and precipitation of dolomite without effecting a volume change in the crystalline portion of the rock. Dissolution and precipitation occur adjacent to one another in conjunction with a narrow aqueous boundary film width which separates the dolomite growth surface from the calcite dissolution surface. The primary condition of replacement is that the volumetric rate of calcite dissolution at the actual replacement site exactly equals the volumetric rate of dolomite precipitation even though the precipitation-dissolution rates in the adjacent pores may differ widely. The volumetric equality of these rates is established by the growing dolomite crystal exerting a linear force of crystallization on the microenvironment of the replacement site. This force increases the pressure within the aqueous boundary film which in turn retards dolomite precipitation and enhances calcite dissolution. The pressure established by this force is that pressure where the thermodynamic potential for dolomite precipitation exactly equals the thermodynamic potential of calcite dissolution times the ratio of the molar volume of dolomite to that of calcite. The kinetic rate of replacement in terms of volume per unit time is proportional to a function of the dolomite thermodynamic potential and inversely proportional to functions of the pressure developed within the boundary film and the size of the growing dolomite crystal. Replacement calcitization of dolomite is an analogous process differing only in direction of the component reactions.

A general mathematical model for balanced global isostasy

A general mathematical model of balanced global isostasy is presented that describes the geometrical relationships among atmospheric, oceanic, lithospheric, and asthenospheric components relative to a fixed, external frame of reference in terms of 19 parameters of mass, density, area, and thickness in five basic equations and various corollaries. The model is applicable and necessary for evaluating lithospheric processes that operate on a global scale or that affect globally oriented parameters such as sea level, freeboard, and ocean basin depth. It also provides a means of testing the internal consistency of a given set of mass, density, volume, and area parameters relative to present crustal geometry by showing how well they combine to predict present continental free-board. With modification, the basic model can be used to examine more complex questions involving glacially induced sea level fluctuations and long-term crustal evolution resulting from differential energy flux to Earth, short-term modulation of heat flux from the asthenosphere, and long-term monotonic cooling.

EVIDENCE FOR A PREHISTORIC PETROGLYPH MAP IN CENTRAL ARIZONA

Abstract For the prehistoric petroglyphs of the southwestern United States, the search continues to correlate these symbols to geographic features, astronomical phenomena or human activity. In central Arizona, a serendipitous discovery of a panel of petroglyphs has led to a potential correlation between the abstract symbols and topographic and location information. This paper summarizes the location, design features and interpretation concerning these petroglyphs. Abstract Para el petroglyphs prehistórico de los Estados Unidos del sudoeste, la búsqueda continúa poner en correlacción estos símbolos a características geográficas, los fenómenos astronómicos o la actividad humana. En Arizona central, un descubrimiento de serendipitous de un entrepaño de petroglyphs ha llevado a una correlación potencial entre los símbolos abstractos y topográfico e información de ubicación. Este papel resume la ubicación, las características del diseño e interpretación con respecto a estos petroglyphs.

Application of turbidite sedimentology to determination of thrust fault displacements in the Carolina Slate Belt

Abstract Discontinuities in the lateral variations of grain size, bed thickness and internal sedimentary structures of ‘classic turbidites’ are used to calibrate thrust fault displacements in the Carolina Slate Belt. The Ugly Creek Thrust, just east of Albemarle, North Carolina, has been determined to have a displacement of 11.7–19.6 km, while the Uwharrie Thrust in the same area has a displacement of 19.6–29.5 km. The method relies on the predictability of lateral change in a down-paleocurrent direction of sedimentological parameters. Values of the various parameters from opposite sides of the thrust fault are cross-plotted against their lateral down-current distance from a common reference point or base line. Data from one side of the fault are shifted relative to that of the other along the lateral distance axis so as to construct by eye a curve that mimics a predetermined curve form for lateral change of the parameter. The amount of shifting necessary to achieve this is equal to the apparent thrust displacement in the down-current direction. This is then trigonometrically related to the actual thrust displacement.

Tertiary non-marine limestone within the strata of the Sullivan Buttes volcanic field, Yavapai County, Arizona: A petrologic and diagenetic investigation

The Oligocene fluvial-volcanic sequence in the Prescott area of central Arizona, USA, contains a thin and laterally limited bed of non-marine limestone. The limestone is characterized by an abundance of opaline diatoms, opalized charophytes, calcified reeds, and cyanobacteria encrustations. Limestone deposition occurred in a small pond of less than a few hundred meters width that formed as the result of damming of a small local stream by a volcanic event. The duration of the pond and limestone deposition was brief and was terminated by burial under volcaniclastic debris. Chemical weathering of coeval volcaniclastic debris was integral both to the formation of the limestone and its syndepositional diagenesis by providing a source for calcium, magnesium, and silica. Biogenicrelated consumption of calcium beyond its rate of re-supply resulted in the elevation of magnesium concentration relative to calcium (Mg/Ca) and the subsequent formation of fibrous calcite cements. Silica concentration seems to be connected to the presence of a series of burrows that provided a pathway from the silica source in the underlying volcaniclastic-rich fluvial sediments to the pond waters. Fluids passing from the sediment through the burrows resulted in the opalization of charophyte internodes and reproductive organs that had collected within the burrow. Silica supplied through the burrows to the pond resulted in diatom growth and their postmortem preservation. Blockage of the burrow system by the growth of cyanobacteria terminated the fluid connection between the sediment and pond water and the subsequent restriction on silica supply resulted in a loss of diatom preservation. The principal depositional phase of the pond was followed by a complex sequence of events that led to the formation of several geopedal fills and isopaceous cements, composed of both calcite and silica.

Modeling sea level changes as the Atlantic replaces the Pacific: Submergent versus emergent observers

We evaluate the sea level changes caused by changing the age/area relationship of the seafloor by altering the thermal structure and hence the elevation of the world seafloor. We do this by having a passive-margined (and therefore continuously aging) “Atlantic-type” ocean open at a uniform rate at the expense of an active-margined (and therefore constant age) “Pacific-type” ocean. This run of the model was specifically constructed to answer the question of how much sea level would change if the Atlantic opened at a uniform rate. Therefore it specifically excludes the other major processes that change sea level. These would include changes in ice volume, other changes in plate generation/consumption rates, continental orogeny and epeirogeny, lithospheric stretching, lithospheric flexure, and thermally induced subsidence at passive margins and amplification thereof caused by water and sediment loading. The isostatic effects of continental flooding and oceanic unloading are accommodated by application of balanced global isostasy to an Earth model consisting of 121 independent columns. The lithospheric portions of the columns are linked to the hypsographic curve. Sea level changes are viewed from two different observational positions: (1) geologically stable and continually emerged continental plateau and (2) geologically stable and continually submerged shelf break. The modeled sea level that results from changing the age/area relationship of the seafloor since 160 Ma peaks at ∼85 Ma and is ∼30% less for the emerged observer (34 m) than for the submerged observer (48 m). Furthermore, the peak is in phase with, but only ∼1/4; (48 m versus 200+ m) of that portrayed by either the “eustatic” curves inferred from seismic stratigraphy or the isostatically readjusted positions of marine strandlines of the stable platform of North America. We therefore conclude that the other 3/4 of sea level rise that accompanies the opening of the Atlantic must be partitioned among the other causes suggested above.