RIPPLE MARK INDICES AND THEIR USES

The following dimensionless parameters (two of them well-known and five of them new) are defined for determination of ripple mark geometry: ripple index (RI), ripple symmetry index (RSI), continuity index (CI), bifurcation index (BI), straightness index (SI), and two different parallelism indices (PI₁ and PI₂). In general, RI = 15 or less indicates wave or water current origin; RI = 17 or more indicates wind or swash origin. RSI = 1.5 or less indicates wave or swash types; RSI = 3 or more indicates wind or water current types. CI = 15 or more suggests wind or wave origin; CI = 10 or less suggests water current origin. BI = 10 or more suggests wave varieties; BI = 1 or less suggests wind varieties. SI = 10² or more indicates wind or deep-water wave types; SI = 15 to 10² indicates wind or wave types; SI =4 or less indicates current types. PI1 = 7 or more suggests wave origin; PI1 = 1 or less suggests water current origin. PI2 = 0.4 or more is probably the result of swash or water current action. PI2 = 0.2 or less is probably the result of wind or wave action. Longitudinal ripple marks (such as rib-and-furrow) and deformed or modified varieties (such as flat-topped tidal-flat ripple marks and nearly- flat-topped intermittent creek ripple marks) have been excluded, inasmuch as (1)they are commonly easy to identify from their appearance, and (2)they are difficult to measure with ordinary methods. Plots of two indices against each other on coordinate paper can be particularly useful; the best combinations are RI vs. RSI, and RI vs. PI1, although several other pairs are almost as good. Where all seven parameters can be obtained, the confidence one can have in the interpretation is close to 98%. The effects of current bias, or depth bias, on wave-type ripple marks, extend to both the symmetry (RSI) and to sediment-transport interpretations. Unless the investigator is reasonably sure that no such bias is present (i.e., RSI = 1.0 instead of some significantly higher value such as 1.5), wave-type ripple marks cannot be used to determine direction of either wave approach or sediment transport. If no such bias is present, wave-type ripple marks still cannot be used to determine precise sediment transport direction. If RSI = 1.0 precisely, it is not even necessary that the ripple crests parallel the waves that formed them. The same restrictions apply to the interpretation of micro-crossbedding (that is, ripple mark internal structure). Despite these seemingly severe limitations, general geometry commonly permits a reliable interpretation, and hence ripple marks can provide a great deal of useful data for paleogeographic interpretations. The swash-zone variety of ripple marks includes two sub-types: those modified by a small but unmistakeable hydraulic jump, and those not so modified. The RI can be used to distinguish between these two, even when they were not observed to form.     

The Composition and Origin of the Connemara Dolomitic Marbles and Ophicalcites, Ireland

The Connemara ophicalcites and associated marbles contain varyingproportions of calcite, dolomite, tremolitic amphibole, serpentinizedolivine, diopside, humite, phlogopite, clinochlore, and quartz.They formed from a chemically precipitated 'primary' dolomitewith a small amount of mica and clay minerals in which muchof the trace and minor element content of the rock, e.g. Al,Fe, Ti, Zn, Ni, Cr, Zr, K, Rb, and rare earths was concentrated.The rock was probably silicified after deposition, possiblyduring metamorphism, and was probably not deposited with majoradmixed detrital quartz or feldspar. The formation during metamorphismof complementary segregated layers rich in either olivine (Fo_98±2)or calcite resulted from important chemical changes controlledby the composition of the stable metamorphic minerals, i.e.those for which solution concentrations exceeded the solubilityproduct for the pore fluid. The mineralogy influenced the localconcentrations of both major and trace elements and emphasizesthe importance of solutions and the stable metamorphic mineralogyin manipulating the composition of some metamorphic rocks. Somecriteria for recognizing segregated layers in metamorphic rocksare given. Serpentinization was probably by addition of water and silicaand not by movement of Fe or Mg. Chemical analyses of forty-threesamples each for twenty-six oxides and elements are given andthe first occurrence of humite in Connemara is reported.     

THE SURF “BREAK”: KEY TO PALEOGEOGRAPHY?

Ancient geosynclinal coastal deposits, preserved and exposed for present-day examination, probably represent interior—and hence protected—shores, and were therefore characterized by low-to-moderate breaker energy levels, as is the modern Gulf of Mexico. Modern low-energy beach sand size distributions are distinguished by an inflection, here called the surf “break”, formed by a winnowing or statistical filtering process, by the breakers, in the finer sizes. The result is a two-segment curve having an intersection in many cases not far from 1.5 phi. If the energy level is very low, a tail of fines may be added, producing a three-segment curve having two inflection points. Along high energy beaches, the surf “break” occurs at such large grain sizes that it cannot be preserved in sand; whether or not it can be recognized in gravel is not known. Ancient sandstones which represent the breaker zone, along a low-to-moderate energy coast, should also exhibit the surf “break”. The Lyons Sandstone, a known Permian beach deposit, provides an example. Other examples are cited. Any useful indicator of the shoreline position, in lithified rocks, should have three characteristics: ease of recognition, high reliability, and strong probability of preservation. The surf “break” scores reasonably well on all three points. Hence it may be an important key to paleogeography. Nevertheless, there still remains a question inasmuch as certain river sands have similar inflections. Whether these kinks were inherited from near-by, but older, beach deposits, or were formed by stream action, is not known. The reliability of the surf “break” is estimated at 80% or better, except for the case where near-shore sands, of whatever age, are reworked briefly by running water.     

Acid Rain and Below-Cloud Scavenging in South-Western China

Major urban areas in south-western China exhibit unique air pollutionproblems due to increasing use of high sulphur-content fuels in an environmentof unfavourable topography and climate. Ambient levels of sulphur dioxideexceed the air quality objectives, and this gas is the major precursor of acidrain. Cloudwater chemistry studies are reported for urban, suburban andcountryside locations, during the period 1985–1989. Although cloudwateracidity was found to increase towards the cloud base, the acidity was muchgreater for rainwater samples collected simultaneously, and was morepronounced in urban rather than neighbouring suburban or countryside regions.The main contribution to the acidity arises from below-cloud scavenging of gasand aerosol and model calculations are able to simulate this behaviour.     

The Composition and Origin of the Connemara Dolomitic Marbles and Ophicalcites, Ireland

The Connemara ophicalcites and associated marbles contain varyingproportions of calcite, dolomite, tremolitic amphibole, serpentinizedolivine, diopside, humite, phlogopite, clinochlore, and quartz.They formed from a chemically precipitated ‘primary’dolomite with a small amount of mica and clay minerals in whichmuch of the trace and minor element content of the rock, e.g.Al, Fe, Ti, Zn, Ni, Cr, Zr, K, Rb, and rare earths was concentrated.The rock was probably silicified after deposition, possiblyduring metamorphism, and was probably not deposited with majoradmixed detrital quartz or feldspar. The formation during metamorphismof complementary segregated layers rich in either olivine (Fo_98?2)or calcite resulted from important chemical changes controlledby the composition of the stable metamorphic minerals, i.e.those for which solution concentrations exceeded the solubilityproduct for the pore fluid. The mineralogy influenced the localconcentrations of both major and trace elements and emphasizesthe importance of solutions and the stable metamorphic mineralogyin manipulating the composition of some metamorphic rocks. Somecriteria for recognizing segregated layers in metamorphic rocksare given. Serpentinization was probably by addition of water and silicaand not by movement of Fe or Mg. Chemical analyses of forty-threesamples each for twenty-six oxides and elements are given andthe first occurrence of humite in Connemara is reported.     

The biotite isograd, Central Pyrenees: a deformation-controlled reaction

The biotite isograd reaction in Cambro–Ordovician pelites from the Garonne dome in the Central Pyrenees involves the production of biotite at the expense of chlorite, and the gradual reduction in the celadonite (Si) content of individual white micas. The mineral assemblages in the biotite-bearing rocks in the Melles area retain abundant evidence of chemical disequilibrium, due to the sluggish nature of major element diffusion within the white micas. The progress of the isograd reaction is strongly controlled by the progressive development of regional Variscan fabrics, and chemical exchange in the white micas in these metamorphic conditions is only possible during active deformation. Chemical resetting of the white micas takes place via the development of compositional zoning in the deforming micas; this probably occurs as a consequence of the introduction of defects and dislocations, causing more efficient diffusion within parts of individual grains. In the absence of deformation, this biotite isograd reaction would take place at significantly higher temperature and be controlled by the relatively high closure temperature of major element diffusion in white micas. Thus, assigning thermal significance to such continuous isograd reactions is impossible without independent constraints: kinetic factors such as deformation may be the dominant influence in many cases, not the thermodynamic controls.     

Little known but important papers on grain-size analysis

Some important papers have apparently gone unnoticed by most sedimentologists, as shown by their absence from bibliographies of recent texts. These papers concern sample size, permissible number of splits, sieving time, and sieve-vs-settling tube comparisons. These papers were published where sedimentologists would not ordinarily see them, but should be required reading for students.     

Progressive Regional Metamorphism of Thin Calcareous Bands from the Moinian Rocks of N.W. Scotland

Bands and pods of calc-silicate rock a few centimetres thickare widely distributed throughout the Late Precambrian Moiniansequence in N.W. Scotland. They probably originated as late-diageneticcalcareous concretions and were subsequently affected by greenschistto upper middle amphibolite facies (Barrovian) regional metamorphismduring the Caledonian (s.l.) orogeny. The calc-silicate rocks described here are from Inverness-shirein the Western Highlands of Scotland. Distinctive prograde mineralassemblages define four narrow zones which run broadly north-southand increase in grade eastwards. Plagioclase composition changesprogressively from albite to near anorthite with increasinggrade and, together with the presence or absence of zoisitebiotite, and hornblende, is the basis of the zonal divisions.Zoisite Ca₂(Al_0.96, Fe_0.04)₃Si₃O₁₂(OH) with biotite is commonin the lower zones and plagioclase ‘jumps’ in compositionfrom calcic andesine to bytownite with the exit of all, or mostof, the zoisite. Similarly biotite-bearing assemblages giveway to those containing ferro-horn-blende and/or pyroxene. Almandinegarnet with approximately 30–40 per cent grossular ispresent throughout and clinozoisite becomes more common in thehigher grade assemblages. Three main episodes of folding (F1–F3) are recognizedin the area, with the calc-silicate assemblages having developedduring the second deformation (MS2) and immediately followingit (MP2). The metamorphic zones (largely of MP2 age) are foldedby major third folds and there is evidence of widespread superimposed‘retrogression’ of probable MP3 age to the east. New analyses of 21 whole rocks by X.R.F., and of 20 mineralsby microprobe, are presented. AI₂O₃ content varies over a smallrange in the whole rock analyses and aluminium is used as astandard for comparing variations in the other elements. CaO/Al₂O₃ratios show little variation but a progressive decrease in Na₂O/Al₂O₃and K₂O/Al₂O₃ with increasingly calcic plagioclase composition(as assessed by measurements on separated fractions), and hence with metamorphic grade, isnoted. Coupled with the fact that the metamorphic zones cross-cutpreviously folded stratigraphic boundaries, this suggests thatboth Na₂O and K₂O have been in part lost from the higher gradecalc-silicate assemblages during progressive metamorphism. Reactions are proposed for the observed mineral changes, andit is concluded that these are most likely achieved by cationexchange through the medium of the pore fluid and result inpartial elimination from the local system of certain elements.No evidence is found that the reaction often quoted as thatresponsible for the exit of zoisite in semi-calcareous rocks,namely 4zo+qtz5an+gross+vap, has taken place.