Skeletal Calcarenites Research Articles

Geoconservation in Sicily (Italy): the Example of the Isola delle Femmine (Palermo)

The Regional Administration of Sicily recently erected Isola delle Femmine—a small island of the Tyrrhenian Sea, close to Palermo—as a geosite. A detailed geological survey has been carried out in order to define the most important geological features of the island together with the development of a new geological map based on topographic data and a digital model at 1:2.000 scale specially processed. Finally, a geological pathway through the island has been traced and illustrated. The geological substrate of Isola delle Femmine consists of a Mesozoic carbonate succession belonging to the Panormide Carbonate Platform. Two lithostratigraphic units have been differentiated. The lowest one consists of dolomitic limestones cropping out in the intermediate and northern part of the island. Despite the absence of biostratigraphic constrains, analogies with comparable deposits from the Palermo Mountains suggested to ascribe this unit to the Upper Triassic. The overlying unit consists of well-bedded rudist and stromatolitic limestones organized in peritidal cycles. The macro- and micro-facies analysis of these Cretaceous limestones allows to attribute this unit to the Lower Cretaceous (i.e., Aptian). Patches of upper Pleistocene skeletal calcarenites rich in benthic foraminifers and calcareous algae overlap the Mesozoic units. Spectacular speleothems such as stalagmites, ray crystals (“raggioni”) of calcite, and mammillary calcite suggest a relative long-lasting exposure of the Mesozoic carbonate substrate to groundwater. This is not surprising since glacio-eustatic oscillations caused sea-level to fall up to 125 m during the Pleistocene thus exposing and linking to the mainland (Sicily) Isola delle Femmine and the surrounding area.

Sedimentary features influencing the occurrence and spatial variability of seismites (late Messinian, Gargano Promontory, southern Italy)

Seventeen layers characterized by soft-sediment deformation structures (SSDS) were identified within the “calcari di Fiumicello”, an upper Messinian (Miocene) stratigraphic unit (30 m thick), cropping out in the northern sector of the Gargano Promontory (Apulia, southern Italy). Facies analysis was performed on the whole outcrop and detailed sedimentological investigations were carried out on the deformed beds, in order to interpret the deformation mechanism, the driving mechanism and the possible trigger agent. Deformed layers occur in some thin-bedded ooidal limestones, skeletal calcarenite, as well as in some pebble-size conglomerate, alternated with marls, deposited in a protected embayment or barrier-island-lagoon system, possibly characterized by high salinity, and tidal influx. SSDS can be classified as load- and slump/slide structures. The continuous exposures allow us to follow a single deformed layer along tens of meters, hence several types of lateral variations were observed that can be summarised as follows: (1) SSDS disappear within a few meters (with a decreasing pattern of their deformation or in an abrupt way); (2) deformed layers laterally change in thickness and morphology; and (3) a single deformed bed can laterally correspond to two deformed beds. Most of the soft sediment deformation features were identified as liquefaction and/or fluidization features related to seismic shocks (seismites). Seismites are often used as an indicator of seismic events, especially along small outcrops, trench excavation and core analysis. This study highlights the value of the sedimentological analysis for paleoseismic investigations, with the aim of improving criteria for identifying seismites in the sedimentary record, and their suitability as marker of seismic events.

Endo‐ and epilithic faunal succession in a Pliocene–Pleistocene cave on Rhodes, Greece: record of a transgression

AbstractA fossil cave and associated sediments and fossil fauna located on the Greek island of Rhodes in the eastern Aegean Sea is reported here, and the depositional history discussed. The sediments were deposited during the late Pliocene, in the interstitial space between basement boulders of up to 1500 tons. The depositional history of the cave comprises eight stages. From initial flooding, the basin experienced a continuous transgression with sea‐level rise in excess of 500 m, followed by a rapid, forced regression of similar magnitude. The recognition of a succession of fossil communities illustrates this transgression, with a seemingly abrupt shift from endolithic to epilithic biota dominance late in the transgressive cycle. The communities recording the increasing water depth from 0 to >150 m are: The Gatrochaenolithes torpedo (bivalve boring) and Entobia gonioides (sponge boring) ichnocoenosis, with peak distribution between 0 and 1 m water depth; the E. gonioides – E. magna ichnocoenosis, with 1–5 m depth peak distribution; the exclusive E. magna ichnocoenosis, with 5–40 m depth peak distribution; and the E. gigantea ichnocoenosis, with a peak distribution approaching 150–200 m. Below this depth, an epilithic community without boring organisms takes over, characterized by the calcareous sponge Merlia cf. normani, and the inarticulate brachiopod Novocrania turbinata. Simultaneously with the succession of the endo‐ and epilithic cave wall fossil communities, skeletal calcarenite accumulated on the cave floor; the erosional remnants of this sediment are insufficient to further expand the overall transgression–regression model.

Caratteri sedimentologici e composizionali delle ghiaie del sintemadi Poggio Martino, Bacino Plio-Pleistocenico di Tarquinia, Italia centrale

The Tarquinia Basin (BARBERI et alii, 1994) in north-western Latium has been filled by a Plio-Pleistocene succession connected to three transgressive depositional cycles. This succession includes the Early Pliocene S. Savino Synthem (circalitoral pelites), followed by the Middle-Late Pliocene Tarquinia Synthem consisting of the two interfingering Macco (skeletal calcarenites and hybrid sands) and Poggio Gallinaro Sands (hybrid sands) members (both foreshore and shoreface deposits) and the Late Pliocene-Early Pleistocene Poggio Martino Synthem member (sands and gravels of shore environment). The last synthem corresponds to the oldest marine terrace, at about 175 m height, among the five Quaternary terraces recognized in the Tarquinia Basin (BOSI et alii, 1990). 646 pebbles have been collected in four sites from the Poggio Martino Synthem gravels, subdividing them into four grain size classes (8-16 mm; 16-32 mm; 32-64 mm; 64-128 mm) by the intermediate axis b. The texture analysis shows that the axis b mean values range between 20,55 mm (site S1) and 33,88 mm (site S4) and the 16-32 mm grain size class includes the majority of clasts. The morphometric analysis, performed by measuring flatness index (CAILLEUX, 1945), dissymmetry index (CAILLEUX, 1945), rounding off index (CAILLEUX, 1945), roundness index (DOBKINS & FOLK, 1970), oblate-prolate (DOBKINS & FOLK, 1970), sphericity index (SNEED & FOLK, 1958) and by detecting the clast forms, suggests that clasts did not undergo a high elaboration, in part due to their lithological characteristics. The compositional analysis allowed to recognize a very high amount of calcareous lithologies (marly limestones, siliceous limestones, calcarenites, quartz rich sandstones, hybrid arenites) coupled with a high frequency of hard and wear resistant clasts like cherts and quartzites. In particular the lower values of roundness, rounding off and flatness have been recognized in sites S2, S3, S4 in which charts and quartzites are more abundant. The texture and morphometric data suggest that the gravels have been transported by streams. The grain size of gravels and the morphometric indexes suggest that streams should have been characterized by a moderate energy and the solid load was deposited in the coastal environment. The gravel source area has been detected in the Mounts Romani sector about 35-40 km NW of the Poggio Martino Synthem area. This sector consists of the Late Cretaceous-Eocene Sillano Formation (GALESTRI AND PALOMBINI of SIGNORINI, 1967) and Tolfa Flysch (unita liguridi; ABBATE & SAGRI, 1970; FAZZINI et alii, 1972; BOCCALETTI et alii, 1987; CIVITELLI & CORDA, 1993), which produced the marly limestones, siliceous limestones, calcarenites, cherts and quartz and mica rich sandstones clasts, of the Triassic Verrucano Formation (ALBERTI et alii, 1970; BOCCALETTI et alii, 1997) from which quartzites are derived, of the Late Messinian Manciano Sandstone (GIANNINI, 1957; SIGNORINI, 1967; PELLEGRINI, 1967a; 1967b; MALESANI & MANETTI, 1967; DESSAU et alii, 1972; FONTANA, 1980a; 1980b; MARTINI et alii, 1995; BARBIERI et alii, 2003) which produced the hybrid arenite clasts and the Late Triassic Calcare cavernoso (ALBERTI et alii, 1970; BOCCALETTI et alii, 1987) immediately SW of the Verrucano Formation which did not produce any supply. The strange lack of Calcare cavernoso clasts in the gravels, despite this formation is widespread on the western side of the Mount Romani till the Argentario Promontory, suggests that the drainage network has not affected the Calcare cavernoso, being confined in a narrow belt of the Mount Romani.

Short-term episodes of carbonate productivity in a Cambrian uplifted rift shoulder of the Coastal Meseta, Morocco

This paper presents an analysis of the spatiotemporal distribution and composition of a volcanosedimentary complex, which has recorded a significant episode of carbonate productivity embedded in a thick and monotonous, turbiditic-dominated rift succession. The studied nucleation of a microbial–skeletal carbonate factory is middle Cambrian in age and situated at Sidi-Saïd-Maâchou, an uplifted rift shoulder of the Moroccan Coastal Meseta located at the junction of several graben segments, where an alkaline basaltic magma was extruded along fissures or intruded as sills and dikes. The geometry and facies of the skeletal calcarenite sheets, composed of hiatal shelled accumulations separated by microbial mats, is comparable to those of the Brèche à Micmacca recorded across the lower-middle Cambrian transition in the Anti-Atlas and High Atlas. The patchy and diachronous distribution of these ‘anomalous’ carbonate factories, spatially and temporally associated with volcanic episodes, marks the northward migration of active rifting structures throughout the Moroccan margin of West Gondwana.

Diagenetic potential for lithification of cool-water carbonate shelf mud

Recent studies of cool-water shelf carbonates have emphasized that lithification of fine-grained carbonates is generally delayed (post-mechanical compaction) due to near-surface aragonite dissolution and greatly reduced effective permeability of mud-size sediment. A more complex response may exist. Texture, mineral, and stable (oxygen, carbon) isotope attributes of sediment offshore of northernmost New Zealand are used as input parameters to understand diagenetic pathways preserved in outer shelf (Te Kuiti Group) and equivalent inner shelf (Torehina Formation) carbonate mudstones of Oligo-Miocene age on North Island, New Zealand. Modern seafloor mud is unlithified, and consists of low-magnesium calcite, < 20% fine-grained skeletal aragonite, and no dolomite. Aragonite is absent in the Te Kuiti Group carbonate mudstones that otherwise consist of low magnesian calcite and trace amounts of authigenic and reworked synsedimentary dolomite. Lithification spanned onset of mechanical compaction in the presence of marine-derived pore waters. For comparison, inner shelf (< 50 m) mudstone of the equivalent Torehina Formation accumulated in a slightly brackish water (c. 31‰) setting. Lithification of carbonate mud was rapid, having occurred in the upper tens of meters in the presence of mixed marine and meteoric fluids, whereas lithification of intercalated siliciclastic mud post-dated mechanical compaction. Near-surface sulphate reduction is recorded isotopically (δ 13C = − 8 to − 15 ‰) by shallow-burial concretions and oyster shell calcite. The comparison of modern and Oligocene sediment suggests that diagenetic potential for lithification of cool-water shelf carbonate mud is driven by diagenesis of in situ and allochthonous mud-size aragonite. Lithification rates may approach those associated with tropical deep-water periplatform (shelf-derived) sediment. The Oligocene carbonate mud lithified faster than regionally intercalated coarser-grained skeletal calcarenites, and likely contributes to deeper-burial intrabasinal diagenetic heterogeneity.

Structural inversion and origin of a Late Ordovician (Trenton) carbonate buildup: evidence from the Tanglewood and Devils Hollow members, Lexington Limestone, central Kentucky (USA)

Throughout east–central United States, Lexington/Trenton limestones are largely Late Ordovician (Chatfieldian; mid-Caradoc), argillaceous, skeletal calcarenites, which are transgressive upward into Late Ordovician (late Chatfieldian–Edenian; mid-late Caradoc) shales and interbedded micrograined limestones. In central Kentucky, however, the same sequence is partially truncated by an unconformity and passes upward into a thicker, younger (Edenian; late Caradoc), regressive shoal complex of coarse calcarenites and calcirudites that is not generally typical of the Lexington/Trenton sequence elsewhere. The shoal complex is interpreted to have been a carbonate buildup and is partially bound by extant faults with basement precursors. Comparison of isopachous maps for pre- and postunconformity buildup units with basement structural lineaments suggests that the buildup is related to reactivation—and in one prominent case, inversion—of the basement structures. The coincidence of structural reactivation, inversion, and buildup development with other major regional, cratonic changes may reflect a reorganization of cratonic, far-field forces accompanying a nearly coeval reversal in the polarity of Taconic subduction.

Early Devonian conodonts from a limestone horizon in the Caballos Novaculite, Marathon Uplift, west Texas

Early Devonian (Lochkovian; eurekaensis Zone) conodonts occur in discontinuous limestone beds in the Caballos Novaculite at five localities in the northwestern half of the Marathon uplift, west Texas. Similar conodont faunas at all five localities indicate that the limestone beds lie at one biostratigraphic horizon within the Caballos. The upper novaculite member directly overlies the limestone horizon at one locality, giving the upper novaculite a maximum age of Lochkovian. The limestone beds are dominantly skeletal calcarenites that represent shallow-water carbonate material transported into a deeper water setting by gravity processes. Restriction of the limestones to the northwestern margin of the uplift and provenance of reworked clasts and redeposited Ordovician and Silurian conodonts suggest a North American shelf source and are evidence of the close proximity of some strata exposed in the Marathon uplift to North America in the Early Devonian. Icriodus gravesi n. sp. is described from the limestone fauna.

Barnacle-dominated limestone with giant cross-beds in a non-tropical, tide-swept, Pliocene forearc seaway, Hawke's Bay, New Zealand

Pliocene, non-tropical, widespread and locally thick (up to 100 m) limestones occur in Hawke's Bay, eastern North Island, where they are intimately associated with very thick ( > 5 km), terrigenous-dominated, Neogene sequences that formed in a tectonically active convergent margin setting. The non-tropical character of the limestones is shown unequivocally by (1) the complete dominance of skeletal calcarenites and calcirudites, (2) the occurrence of oyster banks as the only in situ organic structures, (3) the dominance of barnacles, epifaunal molluscs, bryozoans, echinoderms, foraminifers, brachiopods and calcareous red algae as skeletal components, and (4) the preponderance of calcite over aragonite in the mineralogy of the skeletal grains and cements. The abundance of barnacle fragments in the limestones, and the related exclusive occurrence of only one major organic association, a barnacle-(epifaunal) bivalve-bryozoan assemblage, is striking and unusual given the extent of the limestones. Pecten and oyster valves acted as substrates for barnacle attachment, and their growth was promoted by strong tidal paleocurrents that swept the depositional setting: a long (450 km), narrow (30–50 km) forearc basin seaway, which formed between an actively deforming subduction complex to the east and an uplifting structural ridge to the west. Synsedimentary deformation promoted limestone formation on the margins of the seaway by creating current-swept, clastic-free submarine ridges that acted as the sites of carbonate production. Tidal flows dispersed the carbonate constituents and organised them into a wide spectrum of tide-influenced, cross-bedded and horizontal structures. Most spectacular are occurrences of giant tabular cross-beds, with sets 10–40 m thick and foreset dips of 7–36°, some interpreted as the deposits of major sand bars on carbonate deltas marginal to the mouths of saddles traversing the rising antiforms, and others analogous to modern linear sand ridges. The small- to large-scale planar and trough cross-beds, and the horizontal and lenticular beds that are invariably associated with the giant cross-beds and dominate most sections, represent mainly the deposits of sand waves and sand sheets at inner- to mid-shelf depths in the seaway.

Shallowing-Upward Sequence in a Pleistocene Coral Reef and Associated Facies, San Salvador, Bahamas: ABSTRACT

Excellent exposures of a well-preserved Pleistocene coral reef (dated 120 to 140 thousand years B.P.) extend along the shore northwest of Cockburn Town on the island of San Salvador, which lies at the eastern edge of the Bahamas platform 600 km (375 mi) east-southeast of Miami, Florida. The reef was buried sequentially by nearshore marine and terrestrial carbonate sands during a lowering of sea level. This shoaling produced a vertical sequence of carbonate rocks that represents a progressive change from shallow subtidal through beach to dunel environments. Acropora palmata, A. cervicornis, Diploria sp., and Montastrea annularis are found in growth position up to 2 m (6 ft) above present mean sea level (MSL). Coral rubble, composed principally of A. cervicornis and A. palmata, commonly occurs subjacent to, adjacent to, and up to 80 cm (31 in.) above the in-situ corals. Gullies in the coral rudites are filled by coarse skeletal calcarenites, which in some places contain well-preserved, irregular boxworks of Ophiomorpha sp. and Skolithos linearis burrows. The calcarenites extend upward and outward from the gullies to overlie the corals and coral rudites; lowermost beds commonly have trough cross bedding extending up to 3.6 m (12 ft) above MSL. Upward, the calcarenites become finer grained, skeletal, intraclastic, peloidal, and oolitic, wit gently seaward dipping planar cross beds and beachrock clast breccias occurring up to 4 m (13 ft) above MSL. Fine-grained eolianites with abundant rhizocretions and some paleosols occur higher than 4 m (13 ft) above MSL, and this elevation marks the maximum height of sea level during the development of the Cockburn Town reef. Following this high stand, lowering of sea level produced this fine Pleistocene example of a shallowing-upward carbonate sequence. End_of_Article - Last_Page 539------------

Temperate, meteoric diagenesis of Pliocene skeletal carbonates from eastern England

The Crags of eastern England include marine skeletal carbonates which were deposited and lithified under temperate conditions. The Coralline Crag (Pliocene), in particular, exhibits a variety of carbonate cement types which indicate exposure to meteoric groundwaters during a regressive episode. The Coralline Crag has been divided into an upper division of dominantly well sorted skeletal calcarenites and calcirudites and a lower, poorly sorted silty calcarenite division. ‘Dogtooth’ sparcement and rudimentary syntaxial cement overgrowths on echinoderm fragments are characteristic of the sediments of the upper division, which has undergone extensive aragonite dissolution. This division is consequently relatively evenly cemented. In the underlying sediments aragonite dissolution has not occurred and the sediments have remained largely uncemented, with cement restricted to irregular patches where growth of intra- and inter-particle equant spar has effectively eliminated porosity. In contrast with the upper leached sediments, syntaxial echinoderm overgrowths show a greater development but only where interference from surrounding sediment and sparry cement was absent. The elongate shape of the Coralline Crag outcrop may be the result of preferential cementation of the upper, coarser, well sorted calcarenites originally deposited as an elongate sediment body, possibly as an offshore sandbank.

Ichnology of Pleistocene Carbonates on San Salvador, Bahamas: ABSTRACT

Trace fossils, well preserved and in full relief, occur sporadically in Pleistocene carbonates of intertidal and shallow subtidal origin on San Salvador, Bahamas. Most prominent are irregular boxworks of Ophiomorpha sp., which occur in a subtidal, current-bedded, medium to coarse skeletal calcarenite facies associated with an underlying coral-algal reef facies. Ophiomorpha sp. also occurs in the form of more isolated shaft an dtunnel systems in cross-stratified, coarse Halimeda-rich calcarenites deposited in a tidal delta paleoenvironment. Burrow tues have thick walls (2 to 3 mm, .08 to .1 in.) of micritic material and distinctly mammillated to rugose exterior surfaces; tube outside diameters are 1 to 2.5 cm (.4 to 1 in.). Although Ophiomorpha sp. exhibits an obviously pe leted exterior surface, the pattern of pellet arrangements is not nearly as regular or distinct as that normally found in Ophiomorpha nodosa preserved in siliciclastic sediments. Occurring with Ophiomorpha sp., commonly in abundance, are vertical burrow tubes less than 1 cm (.4 in.) in outside diameter and with lengths of up to 15 cm (6 in.). These tubes are assigned to Skolithos, and two or more types are present. Rhizocretions formed of calcrete and presumably initiated by the action of plant roots occur commonly in most facies on San Salvador, and they can easily be mistaken for trace ffossils of invertebrate origin, particularly Ophiomorpha sp. Criteria for distinguishing Ophiomorpha sp. from rhizocretions include the following. (1) Ophiomorpha sp. has a distinct lining of regular thickness, and individual segments of the burrow system have consistent diameter; rhizocretions do nto have a distinct lining and are irregular in diameter. (2) The interior surface of Ophiomorpha sp. is smooth and the exterior surface distinctly mammillated; rhizocretions have highly variable interior and exterior surface. (3) Ophiomorpha sp. compelxes have much more consistent patterns of shaft/tunnel arrangement than exhibited by rhizocretion systems. Calcarenites of beach facies are widespread along the coastline of the island, but these facies do not contain Ophiomorpha sp. In few places, these facies have unlined vertical burrows of variable diameter and trails, both attributable to the activity of crabs. The modern marine carbonate environments surrounding SanSalvador exhibit much trace-making activity and provide analogs for further interpretation of the Pleistocene trace fossils and their associated paleoenvironments. End_of_Article - Last_Page 445------------

The Coral Reef of Nijar, Messinian (Uppermost Miocene), Almeria Province, S. E. Spain

ABSTRACT The Nijar Reef is a thoroughly exposed and representative example of the Messinian coral reefs outcropping along the periphery of the western Mediterranean. Facies patterns in the Nijar Reef Complex are presented in this paper as a generalized guide or model for the study of other Messinian reefs. Messinian paleogeography of this area was characterized by an archipelago whose islands are present-day Sierras. At the beginning of the Messinian and before development of the reefs, marine terrigenous sediments (Marginal Terrigenous Complex) were deposited on an irregular erosion surface on the metamorphic basement. The Reef Complex developed as a belt, fringing the positive areas (islands) when terrigenous deposition decreased. The Reef Complex consists of: 1) reef-core, constructed by a framework of vertical, branching colonies of Porites coated by laminated micrite (probably submarine cements), which form pinnacles as well as thickets; and 2) fore-reef, made of sloping beds of broken skeletons and debris. The fore-reef is subdivided into: i) reef-talus slope, ii) proximal slope, and iii) distal slope. The reef talus forms massive strata with dips of 20-30° and abundant pebble-to-boulder size slumped blocks of reef-core and coral breccia. The proximal slope shows dips of 10-20° of well-bedded, thick strata with internal parallel layering of granule to sand-size skeletal calcarenites. The distal slope is made of predominant bioturbated skeletal calcisiltites and fine-grained calcarenites with ngles lower than 10°. The distal slope grades into the basinal pelagic marls of the Nijar plains. The original reef morphology is well-preserved in present-day topography, with relicts of pinnacles, reef valleys, and slopes. Messinian volcanic highlands located to seaward complicated the local paleogeography with atoll-like features. Reef growth was concomitant with a gradual fall of sea level and occurred before the Messinian gypsum was deposited on top of the basinal marls in what is now the plains area. The Reef Complex is partially truncated by a major erosion surface, and it is overlain by oolitic shoals and stromatolites of the so-called Terminal Carbonate Complex. The Terminal Complex is interpreted as an upslope equivalent (in age) to the Messinian Upper Evaporite, and it implies a new cycle of sea level rise.

Microdolomite-Rich Syntaxial Cements: Proposed Meteoric-Marine Mixing Zone Phreatic Cements from Mississippian Limestones, New Mexico

ABSTRACT Inclusion-rich echinoderm-syntaxial cements comprise up to 38% of the intergranular cements in Mississippian skeletal calcarenites of the Lake Valley Fm., N.M. These cements have crystal morphology, substrate selectivity, and luminescence characteristics identical to previously interpreted fresh water phreatic cements, yet contain microdolomite inclusions, implying that they were originally Mg calcite. These petrographic characteristics, plus the restriction of the microdolomite-rich syntaxial cements to the lower part of the Lake Valley Fm., argue for their having precipitated in the zone of mixing between meteoric and marine phreatic groundwaters. Petrography of these cements, their association with interpreted fresh water phreatic cements, and their regional restriction to the south, suggest that the mixing zone cements were formed mainly daring the slowing down stages of a late Mississippian global sea level drop. These cements plus other microdolomite-rich calcites reported on elsewhere indicate that coarse crystals of Mg calcite commonly act as closed or semi-closed systems during re-equilibration to low-Mg calcite, rather than undergoing complete loss of Mg through conventional incongruent dissolution.

Stratigraphy and structure of La Encantada Mine area, Coahuila, Mexico

At La Encantada, three mines produce Pb-Ag ores from deposits that occur in fracture zones in the Cretaceous Aurora limestone, within the north-northwest–trending Sierra Madre Oriental of northern Mexico. The host limestone is remarkably monotonous, almost totally lacking in diagnostic marker beds. The limestone unit is gently folded into a major anticline, the Sierra de La Encantada, which is cut by major frontal strike faults and dissected into blocks by cross-fractures and faults. Because of the uniformity of the limestone (apart from micro-fossil assemblages), the area was studied by repeated sampling, thin sectioning, and detailed lithofacies and biofacies identification. A very high proportion of all thin sections contained diagnostic or partially indicative fossils. Regional mapping, begun in the field by recognizing only two rock units, was accordingly refined by laboratory analyses, and ten subdivisions of the local sequence were ultimately traceable. These units were based on local refinements of F. Bonet's planktonic biozone scheme for the Cretaceous System of eastern Mexico. In consequence, the areas on the western flank of the anticline, where ore deposits are known, can be more precisely related to the prevalent step faulting and flank grabens of the region. There is a preferential association of regional-scale ore-bearing fractures with sparry calcite–cemented, skeletal calcarenites. All the ore worked to date has been from heavily iron-stained, secondary, oxidized ore of complex composition. The primary ore is likely to have been argentiferous galena with sphalerite, magnetite, pyrite, and proustite. It occurs as oxidized remnants in two high-grade chimneys, originally with about one million tons in La Prieta and 350,000 tons in El Escondida; in addition, 500,000 tons occurred in a series of veins and small mantos. Recent work has uncovered a zone of lower grade but extensive sulfide tactile ore, possibly associated with the contact zone of an intrusive mass that occurs at depth. A halo of contact metamorphism, marmorization, and bleaching surrounds the ore. The alteration halo is so variable in its development as to be of little value in the search for more ore. The most important ore-controlling structures appear to be the northeast-trending cross-fractures with which the chimneys and veins are associated. Major faults do not appear to be mineralized, presumably because they have been resealed with fault gouge. Fractures without apparent movement of the opposing surfaces and, therefore, of minor tectonic significance, at surface at least, seem to retain their dilatancy long enough to allow mineralizing fluids to surface.

Facies and Related Reservoir Characteristics Golden Spike Reef Complex, Alberta

ABSTRACT The stratigraphy, facies and porosity of the Golden Spike reef complex, an isolated buildup 575 ft (175 m) thick and less than 2.5 mi (4 km) in diameter, was studied from cores and wireline logs of 10 wells. The total build-up comprises the Late Devonian Leduc Formation (Frasnian) which is divisible informally into lower, middle, and upper Leduc. The middle and upper Leduc are each divisible into two major divisions. Fore-reef strata consist of detrital coral and tabular stromatoporoid rocks; reef strata comprise the massive stromatoporoid facies; skeletal calcarenites represent the principal reef-flat facies while laminites, dolomitic mudstones, and Amphipora mudstones are locally present in the interior. The principal subdivisions of the Leduc Formation represent three major depositional events: 1) basal bank or biostrome stage -- lower Leduc (Cooking Lake equivalent), 2) biohermal stage -- middle Leduc, and 3) reef-fringed bank stage -- upper Leduc. The lower Leduc represents a transgressive bank with mostly mud-supported carbonates. The base of the middle Leduc (Division 1) forms extensive boundstones and reef frameworks which grade progressively into a shallow-marine shoal phase (Division 2) with less extensive reef frameworks on the margin. In the upper Leduc (Divisions 3 and 4) a narrow discontinuous reef zone is on the margin with intertidal laminites and mudstones predominating in the interior. This sequence can be correlated with known depositional events at Redwater and other Leduc-age reefs. Porosity distribution is facies-dependent and related to depositional conditions for each stage of development. Highest porosity and lowest irreducible water saturation occur in grain-supported subtidal massive stromatoporoid and skeletal calcarenite rocks. Nearly all the pore space in these rocks is available for hydrocarbon saturation, and high production can be maintained with little water. Rocks lacking interparticle porosity, including intertidal and supratidal carbonates in the interior and fore-reef coral and tabular stromatoporoid carbonates, have low effective porosity and high irreducible water saturation, and are generally poor reservoir rocks. The best porosity is developed near the margin during mildly transgressive stages of reef-complex development (Divisions 2 though 4).

Winterset Algal-Bank Complex, Pennsylvanian, Eastern Kansas

Algal banks are widely distributed, are important in late Paleozoic carbonate provinces, and commonly are good reservoirs for accumulation of hydrocarbons. The term as used in this report indicates an unusually thick sequence of carbonate mudstone built by in-place organisms in an environment of low turbulence. The bank stood slightly above the surrounding sea floor. The Winterset Limestone bank (Dennis Formation, Kansas City Group, Pennsylvanian) is a complex, biogenic accumulation associated with a variety of rock types. The dominant bank-forming organism is a red alga, Archaeolithophyllum, which flourished in a quiet-water environment. Environmental factors reflected in lithologic-biota characteristics are light penetration, turbidity, turbulence, influx of terrigenous clastics, and changes in sea level. The westward thickening of Winterset Limestone in the subsurface is attributed to a more extensive development of the algal-bank complex. The algal bank influenced biota distribution and sedimentation by providing a shallow platform on which other environments could develop. Later, during deposition of the upper Winterset unit, the algal bank acted as a barrier against influx of terrigenous clastic sediment from the south and restricted circulation of water to the north; this may have caused an increase in salinity. Limestones of the Winterset comprise seven major rock types: skeletal mudstone, algal-bryozoan boundstone, skeletal calcarenite, oolitic calcarenite, laminated mudstone, algal-fusulinid mudstone, and gastropod-fusulinid mudstone. The skeletal mudstone is interpreted as an off-bank accumulation in the deepest water area or basin. The algal-bryozoan boundstone records a quiet-water environment or marginal shallow shelf. The skeletal calcarenite represents a shoal environment intermediate between that of the algal-bryozoan boundstone and oolitic calcarenite. The latter indicates areas of agitation above a shallow-water bank. The laminated mudstone was deposited in an intertidal to supratidal environment. The algal-fusulinid mudstone represents an environment intermediate between the upper algal-bryozoan boundstone and the gastropod-fusulinid mudstone, which accumulated in slightly deeper water than the algal-bryozoan boundstone. The lack of diverse marine organisms suggests that environmental factors other than d pth were of primary importance for this facies. Other than lithification, dolomitization was the major diagenetic process in the Winterset Limestone. Dolomitization was selective, replacing (1) drusy calcite mosaic in the algae, (2) carbonate matrix adjacent to algae, and (3) fossils which originally were composed of aragonite. Concentration of the magnesium ions was prior to deposition of the upper Winterset unit. The subsequent dolomitization seems to have been controlled, at least in part, by algal-bank development that caused a migration of magnesium-rich waters downward and laterally through the porous ooid sands into the algal bank. Penecontemporaneous dolomite formed northward along the southern part of the supratidal environment, whereas secondary dolomitization occurred farther south. Dolomitization may result from either concentration of magnesium from coralline algae, by the seepage refluxion model, by the Dorag model, or a combination of the three.

Pennsylvanian stratigraphic reefs in Kansas, some modern comparisons and implications

Broad platformlike buildups in the Stanton Limestone in southeastern Kansas are composed primarily of lime mud and phylloid algae and are rimmed with skeletal calcarenite composed largely of echinoderm and algal debris. Bordering the buildups are large (1×30 km) channels, lined or filled with similar calcarenite, and a broader basin containing shaly skeletal calcilutite. Grain abrasion and spar cement in the rimming calcarenites indicate water agitation and suggest wave resistance of the buildups during growth. Wave resistance may have been provided by 1) stalked echinoderms, which are resilient when alive and bind sediment with their roots, and also by 2) early drusy cementation of loose sediment to a coherent mass. Comparison to modern channel-separated buildups in the Persian Gulf and the Great Barrier Reef complex brings out the relative significance of organic frameworks in these different geologic settings. Recognized organic frameworks range from absent or insignificant in the Pennsylvanian examples through locally present but insignificant in the Persian Gulf, to apparently important in maintaining the buildups in the Great Barrier Reef. All three examples, however, exhibit patterns of original hydrodynamic control over initiation and gross form of the buildups. Determining significance of organic frameworks to overall buildup growth involves examining both their positions relative to evidence of wave action and the nature of binding in contemporaneous talus produced by wave action on the buildup.

STratigraphic Petrology as Aid to Mapping Calcarenite Bodies in Lower and Middle Trinity Cretaceous Rocks of San Marcos Platform, Texas: ABSTRACT

Lower Trinity rocks (Sycamore sandstone, Hosston sandstone, and Sligo limestone) and middle Trinity rocks (Hammett shale and Cow Creek limestone) represent two depositional cycles, bounded by regional disconformities, in the basal Lower Cretaceous section of south-central Texas. The cyclic units contain alluvial sandstone, alluvial and lagoonal terrigenous mudstone, lagoonal carbonate mudstone, and oolitic and skeletal calcarenites. Study of outcrops, cores, well cuttings, and electric logs demonstrates that these lithofacies are in ordered belts parallel with paleoslope contours on the depositional shelf. Calcarenites formed where highly agitated marine water impinged on the shelf surface. At different times, belts of calcarenite were formed at the shoreline, about 50 mi offshore, and 75-90 mi offshore. The resulting bodies are 5-25 ft thick, 5-10 mi wide, and tens of miles long. The approximate location of each offshore body can be determined by mapping lithofacies belts landward. Seaward from the calcarenite bodies, ooliths and well-rounded skeletal fragments are present as exotic grains in mud. These round grains range from 0.05 to 0.1 mm in diameter and are in quiet-water, open-marine sediments characterized by calcareous mud, terrigenous silt, glauconite silt, large angular shell fragments, and fragile whole fossils. Careful correlation of electric logs on the basis of scattered cores, plus thorough examination of well cuttings and cores, provides the ability to predict locations of calcarenite belts within these cyclic deposits. End_of_Article - Last_Page 600------------

Reciprocal Deposition within Niagaran and Early Cayugan (Silurian) Carbonates and Evaporites, Northern Michigan Basin: ABSTRACT

Time-stratigraphic relations within Silurian strata of the northern Michigan basin provide a model for reciprocal deposition of carbonates and evaporites. The Niagaran of the basin interior consists of crinoidal hematitic biomicrites. Toward the basin margin the biomicrites thicken and lose their hematitic character. A belt of pinnacle reefs marks the approach to the basin margin. The Niagaran pinnacle reefs have a lower crinoidal zone and an upper coral-algal zone. At the basin margin the Niagaran thickens abruptly into a dolomitized barrier reef complex. The barrier reefs were constructed mainly by corals and massive stromatoporoids which prograded basinward over thick skeletal forereef calcarenites. Niagaran barrier and pinnacle reef construction was halted in the early Cayugan by an episode of evaporite deposition. Karst features within Niagaran carbonates suggest subaerial exposure of the barrier and pinnacle reefs at this time during a period of lowered sea level. Return of high sea level caused cessation of evaporite deposition and rejuvenation of the pinnacle reefs; also, fringing reefs developed along the margin of the former barrier reef trend. However, the renewed reef development was of a considerably different biotic character. These early Cayugan reefs possess a lower massive encrusting algal zone and an upper laminar stromatoporoid-stromatolite zone. Corals are notably absent. Reef development again was halted by evaporite deposition followed by another episode of car onate deposition generally devoid of reef rejuvenation. End_of_Article - Last_Page 2109------------