Calcareous Crust Research Articles

Calcareous crust (caliche) profiles and early subaerial exposure of Carboniferous carbonates, northeastern Kentucky

ABSTRACTSequences of laminated limestones found within thin Carboniferous carbonate strata of northeastern Kentucky were studied to determine their origin and palaeo‐environmental significance. These laminated zones are strikingly similar to Holocene and Pleistocene surficial calcareous crusts (caliche) profiles that occur in various parts of the world.Carboniferous laminated carbonates are associated with shallow marine carbonate units, palaeokarst, and overlying palaeosol zones. A typical laminated profile ranges in thickness from 1 to 2 m and contains brecciated, light olive‐grey to brown micrite that lacks distinctive bedding. Structures and textures common in most profiles include: (1) calcareous and silicious laminae (laminae form diffuse, alternating light and dark bands that generally parallel bedding but often fill fractures and vugs within the rocks); (2) particles (allochems, and micrite and microspar fragments) coated by brown microcrystalline calcite; (3) brecciated texture; (4) circular to elliptical fossil moulds (occur in sinuous patterns and fill fractures within the rocks); (5) large and small scale fracture patterns.Subaerial weathering and vadose diagenesis of carbonate mud banks or islands is suggested as a mechanism for the formation of these Carboniferous calcareous crust profiles. These ‘crusts’ formed by a combination of solution (karsting), brecciation, and soil development that transformed an exposed marine biomicrite (‘host’ rock) into a porous subsoil rubble. Laminated ‘crusts’ and coated particles developed as the result of dissolution and reprecipitation of CaCO3 and SiO3 from the soil and carbonate bedrock.Carboniferous laminated carbonates in northeastern Kentucky are often referred to as ‘algal limestones’ because of their superficial similarity to some modern and ancient algal structures. This study, however, reveals numerous characteristics that can only be explained by diagenesis in a subaerial environment.

Holocene and Pleistocene Calcareous Crust (Caliche) Profiles: Criteria for Subaerial Exposure

ABSTRACT Surficial, calcareous crust profiles, hard, irregular, subhorizontal, calcareous laminae, surrounded by crumbly, chalky carbonate, form at the surface of many limestone successions in semi-arid areas. Detailed study of Holocene profiles, developed in late Pleistocene reef limestones on northern Barbados, reveals that diagenesis of limestone in such profiles creates a diagnostic, repeatable set of textures and fabrics. The original limestone is altered by brecciation, recrystallization (to microspar), micritization and boring. The CaCO3 thus put into solution and augmented by CaCO3 from fallout and salt spray is reprecipitated as calcite (less than 4 mole percent MgCO3), in the form of crystals often, quite different from those in the vadose zone below. Calcite crystals are precipitated as flower spar, micrite, random needle fibres, and tangential needle fibres. Several of these crystal morphologies are similar to those precipitated from highly supersaturated solutions or solutions that contain appreciable amounts of other dissolved ions. These 4 basic crystal types are not only precipitated as void-filling cement, but, more important, combine to form characteristic structures such as oolith-like coated particles, pelletoids, and the laminar crusts themselves. The diagenetic textures and fabrics described above are also the basic units of many Florida and Middle East calcareous crust (caliche) profiles, suggesting that the above features may be characteristic of calcareous crust profiles in general. Fossil calcareous crusts and associated features, buried in the Pleistocene limestone succession on northern Barbados without alteration, indicate that these features may well be preserved as indicators of subaerial exposure in the fossil record.

Age of Asian Tektites

Nannofossils in a calcareous crust firmly attached to a tektite dredged from the South China Sea, indicate a minimum age of about 1 m.y. for indochinite tektites on the basis of a 3-m.y. duration for the Quaternary Period.

NOTES ON MYXOMYCETES VIII

Specimens of a Didymium collected at Endegeest near Oegstgeest, a suburb of Leiden, on holly leaves, were put aside by Prof. Dr. W. K. H. Karstens as being near to Didymium squamulosum (Alb. & Schw.) Fries, but not identical with it. Some of the specimens were collected in August 1944 by Dr. S. J. van Ooststroom, whereas several other ones were collected in October of the following year by Prof. Karstens at the same locality; they are all very similar, and remarkable in the smooth white calcareous crust, which is distant from the membranous inner part of the peridium, and in the rather dark spores, which are nearly all encircled by a thin, sometimes fragmentary ridge. Comparison with a large number of specimens of D. squamulosum has convinced me that the specimens collected at Endegeest are indeed distinct from that species. LISTER, in a footnote to D. squamulosum (3rd ed. 1925, p. 118), mentions a form collected on holly leaves, but the description and figure prove that this is plainly D. squamulosum, and certainly not identical with the above mentioned specimens. The specimens from Endegeest are not identical with D. praecox de Bary either. The latter is described by Lister “as so inconstant that the name cannot be applied to mark even a variety”. However, D. praecox was described by Berlese in Saccardo (Syll. 1306) and by Massee (Mon. p. 223) (the two descriptions, probably based on that given by Rostafinsky, which was not seen by me, are practically identical) as possessing a double peridium. Study of a duplicate of de Bary’s type specimen in the Rabenhorst “Fungi Europaei” collection no. 367, 1861, preserved at the Rijksherbarium at Leiden (no. 910243-676), shows this to be D. squamulosum, as the crystalline lime crust closely adheres to the membranous inner layer of the peridium, a condition which is characteristic of this species; this is seen quite clearly at the time of dehiscence, as the two layers break away simultaneously. The spores were found by me to be 10-11 µ in diam., and not 8—9 µ, as they are said to be in Massee’s description (which, however, comes within the range allowed for the spores of this species by Lister and by Martin in their monographs, viz. 8-11 µ), and they are spinulose; some of the dark spinules are grouped in clusters, whereas the remaining ones are unevenly and sparingly scattered between these clusters. In the specimens collected at Endegeest the crystalline lime layer of the peridium, as stated above, is distinctly separated from the membranous inner layer, the latter, moreover, is often provided with light brown areolae, a feature which is seen also in D. nigripes and in D. melanospermum, but which I myself have not met with in D. squamulosum. However, Lister describes the inner peridium of the latter as “sometimes mottled with red-brown towards the base”; this, therefore, is a point which deserves further study. Other noteworthy points are that the spores of the new species are provided with a ridge and that the spinules are not arranged in clusters.

III. Observations on the changes which have taken place in some antient alloys of copper. By John Davy, M. D. F. R. S. In a letter to Sir Humphry Davy, Bart. Pres. R. S

Permit me through you to lay before the Royal Society, the results of some experiments and observations on the incrustations of certain antient alloys of copper, which I trust may not he undeserving of notice, whether considered in connexion with the arts of Antient Greece, or in relation to the slow play of chemical affinities acting during a long period of time. I shall commence with the most interesting object that I have examined since I have been in the Mediterranean, for which I am indebted to His Excellency Sir Frederick Adam, His Majesty’s Lord High Commissioner in the Ionian islands. This is a bronze helmet of the antique Grecian form, recently found in a shallow part of the sea, between the citadel of Corfu, and the village of Castrades. Both internally and externally it is partially encrusted with shells, and a deposit of carbonate of lime. The surface of the helmet generally, both under the incrustation, and where freed from it, is of a variegated colour, mottled with spots of green, dirty white, and red. On minute inspection, the green and red patches exhibit a crystalline structure, and the red very distinctly; and on examining them with a lens, they were found to be formed of octohedrons of the red oxide of copper, intermixed with crystals of the same form of metallic copper. These crystals are most distinct at the surface; they even adhere to the calcareous crust, and may be detached with it, pre­senting well-formed facets towards the part from which they have been separated. Chemical examination shows that they are actually composed of metallic copper, without the least tarnish, and of the pure ruby protoxide of copper; and that the green rust consists principally of carbonate and sub-muriate of copper, and the dirty white chiefly of oxide of tin. The mineralizing process (if I may be allowed the expression) which has produced these new combinations, in general has penetrated very little into the substance of the helmet. The incrustation and rust removed, the metal is found bright beneath; in some places considerably corroded, in others very slightly, It proves on analysis to be copper alloyed with 18.5 per cent, of tin. Its colour is that of our common brass, and it possesses a considerable degree of flexibility.