Interior Facies Research Articles

Facies and Geologic History of an Exposed Miocene Rift-Margin Carbonate Platform: Gulf of Suez, Egypt

Abu Shaar is a small exhumed carbonate platform of lower to middle Miocene age on a horst of Precambrian crystalline rock along the western side of the Gulf of Suez. The reef-rimmed complex is entirely dolomite but with exquisitely preserved primary and diagenetic microfabrics. Similar platforms, some of which contain hydrocarbons, are buried beneath evaporites in the Gulf. The platform developed in three stages, defined as local members of the Rudeis Formation. The first stage records deposition during progressive marine onlap as aprons of mixed terrigenous-carbonate sediments evolved into a wide reef-rimmed platform. Synsedimentary platform-margin collapse subsequently removed the most seaward portions of the reefs along the eastern Gulf-facing margin. These truncation surfaces were then covered by a second phase of reef growth and deposition of fore-reef carbonates. The third and final stage, deposited following a sea level drop and subaerial exposure, is a series of well-bedded peritidal carbonates that become progressively more evaporitic upward. Fore-reef facies are again truncated by synsedimentary slope failure. Former evaporites are now recorded as collapse breccias, contorted bedding, black chert nodules, and evaporite molds. Porosity is mainly secondary as vugs and molds and is highest in platform interior facies. These more porous sediments contained more aragonitic components originally and were less affected by submarine cementation. In contrast, the higher degree of synsedimentary lithification of platform-margin reef and fore-reef sediments is comparable with what is commonly observed in modern reef-dominated platform margins.

Quantitative Paleobathymetry of Early Proterozoic (1.9 B.Y.) Continental Slope, Rocknest Formation, Wopmay Orogen, N.W.T., Canada: ABSTRACT

The Rocknest Formation is an early Proterozoic (1.9 b.y.) westward-facing, rimmed carbonate shelf that evolved from a ramp developed on quartzites of the underlying Odjick Formation. Shelf interior facies are cyclic peritidal dolomites; shelf edge facies are reefal stromatolitic boundstones and intraclast/ooid grainstones, and slope facies are turbidites, slope breccias, and shelf edge breccias. The carbonates are overlain by black shales and graywackes of the Recluse Group. Facies that represent the shelf edge-to-slope transition of the Rocknest Formation are well exposed for almost 200 km (125 mi) along the strike length of the major north-south trending syncline. At the southern end of the syncline, the Rocknest Formation comprises 230 m (755 ft) of slope deposits on the west limb and 660 m (2,165 ft) of shelf edge deposits on the east limb. The difference in the decompacted thickness of the Recluse Group on both sides of the syncline, as measured from the top of the Rocknest Formation to a marker bed in the Recluse, is 590 m (1,935 ft), which represents the total paleobathymetric relief. However, the difference in thickness of the Rocknest Formation, 430 m (1,410 ft), is less than the total paleobathymetric relief, which indicates that most, but not a l of the total paleobathymetric relief was produced by constructional aggradation of the shelf edge. The remaining relief, 160 m (525 ft), was inherited from a preexisting slope at the top of the Odjick Formation and therefore represents the initial relief of the Rocknest slope. Average inclinations of the Rocknest slope can be calculated given the distance between sections (approximately 4,500 m, 15,000 ft). The initial average inclination of the slope is 2°, and the final average inclination of the slope is 7°. The sedimentologic and stratigraphic evidence strongly suggests a depositional model involving a ramp that evolves into a steep, marginal escarpment passing seaward into a shallow-dipping slope. As such, the geomorphic development of the Rocknest slope is considered to be analogous to the slopes of many Phanerozoic rimmed shelves, implying that similar evolutionary processes were operative during the early Proterozoic. Figure End_of_Article - Last_Page 475------------

Early Cretaceous Drowning and Recovery of a Carbonate Continental Margin, Eastern Arabia: ABSTRACT

The eastern margin of the vast Mesozoic carbonate platform which underlies much of Arabia is exposed in the mountains of Oman and the United Arab Emirates. Outer platform and margin facies occur as autochthonous windows in the Oman Mountains and as allochthonous terrane in the Musandam Mountains to the north. These strata are in turn surrounded by westward-thrust nappes of contemporaneous deep-water strata and oceanic lithosphere. Inundation or drowning of the platform occurred in earliest Cretaceous time and is reflected by Jurassic reef limestones overlain directly by Lower Cretaceous pelagic carbonates. In Oman the outer platform strata are buried by over 250 m (820 ft) of these radiolarian lime mudstones. The abrupt upward transition in middle Cretaceous time to ooid grainstones and then muddy fossiliferous limestones interbedded with rudist biostromes documents buildup to sea level and eastward progradation of first high-energy facies and then lower energy interior facies. In the Musandam Mountains to the north, however, where the Jurassic margin proper was drowned, deep submergence was accompanied by flexure and the Lower Cretaceous limestones are slope deposits as indicated by numerous turbidites, debris flows, and slope unconformities. These strata are also transitional upward into shallow water facies so that by Middle Cretaceous time the high-energy facies were once more at the shelf edge. These events, which are reflected along the length of this extensive continental margin, occurred during a time of apparant eustatic sea level rise. End_of_Article - Last_Page 549------------

Isotopic composition and diagenetic history of carbonate cements in Devonian Golden Spike reef, Alberta, Canada

Extensive cementation in the margin of the Golden Spike (Upper Devonian, Alberta) reef complex is due largely to formation of carbonate cements in a marine environment. Subaerial cements are absent in the reef margin, but they occur in shallow-water reef interior facies. Middle and late burial calcite cements are less abundant but occur vertically and laterally throughout Golden Spike. The δ 13 C and δ 18 O values of selected carbonate cements and sediments, mainly from the reef margin, show: (1) a narrow range (+4.0‰ to +1.6‰) of δ 13 C values for all samples; (2) the most positive (X = +3.8‰) δ 13 C values occur in lime mudstones; (3) radiaxial calcite cements (submarine) and associated (often inter-layered) marine internal sediments have similar δ 13 C (+3.5‰ to + 1.6‰) and δ 18 O (−5.8‰ to −7.7‰) values; (4) submarine radiaxial and radial fibrous calcites (X = −6.7‰), middle burial non-ferroan calcites (X = −8.8‰), and late burial ferroan calcites (X = −13.0‰) contain progressively more negative δ 18 O values. Textural evidence (that is, cement fabrics, their distribution, superposition, and relationships with associated sediments) and isotopic compositions considered in light of the Golden Spike burial history indicate the following. Submarine cements of the reef margin apparently have not been significantly modified by meteoric waters ( 18 O, 13 C depleted) throughout their diagenetic history. Submarine cements and associated sediments have δ 13 C values similar to modern marine cements and sediments, and any diagenetic re-equilibrations occurred in a “closed system” with little or no addition of 12 C. δ 13 C values in middle and late burial stage cement are the result of calcite precipitation, probably in equilibrium with subsurface fluids. The 18 O/ 16 O ratios for marine sediments, fossils, and submarine cements are probably due to isotopic re-equilibration of originally enriched (marine) 18 O/ 16 O at elevated burial temperatures before the end of the Mississippian. On the basis of burial history, middle and late burial calcite cements are post-Mississippian to pre-Late Cretaceous; the Late Cretaceous is the most likely time of major petroleum entrapment.

Secular Oscillations of the Geomagnetic Field Recorded by Sediments Deposited in Caves in the Mediterranean Region

Summary The palaeomagnetism of a 2.4-m section of interior facies sediments from the Upper Dry Cave, Jeita, Lebanon, reveals inclination oscillations which can be correlated with those recorded 1000 km away in core 1474 from the Black Sea. The ' magnetic ' ages so obtained for the Jeita section are consistent with the age (0–16 000 BP) deduced from anthropological and palaeoclimatic evidence. Entrance facies sediments have been studied from a 3.4-m section in Arbreda Cave and from a 1.1-m section in Hermits' Cave, Catalonia, Spain. The age of the Arbreda section, deduced from anthropological and palaeoclimatic controls is between 35 000 BP and 50 000 BP and the Hermits' Cave section is Middle Palaeolithic. At Arbreda, declination oscillations are observed and abnormally low inclinations are recorded at two horizons one of which may correlate with a geomagnetic excursion reported in an Indian Ocean core at about 40 000 BP. The palaeomagnetic record from Hermits' Cave exhibits large amplitude swings. The overall pattern of the results obtained from this and other studies is discussed in terms of a model field in which the long period secular variations originate from oscillations of fixed radial dipoles located in the outer core in the positions computed by Alldredge & Hurwitz.

Flow differentiation within leopard rock dikes, Bighorn Mountains, Wyoming

Locally along their strike, several mafic dikes from the Bighorn Mountains contain leopard rock interiors which consist of closely-spaced, commonly ellipsoidal or subspherical, cracked and resorbed plagioclase (An 64–75) phenocrysts typically 5–8 cm in maximum dimensions. The leopard rock groundmass is petrographically and mineralogically identical to the nonporphyritic tholeiitic dolerite which occurs as uniformly thick tabular bodies on both flanks of the leopard rock interior. The boundary between the flanking and interior facies is marked by a rapid but gradational increase in phenocryst concentration toward the dike center. The data indicate that leopard rock is formed by axial segregation of calcic plagioclase phenocrysts within a single body of ascending phenocryst-rich magma via flow differentiation. Flow differentiation is probably related. to the creation of a grain dispersive pressure due to mechanical interactions between phenocrysts.

Metamorphosed Mafic Dikes from the Southern Bighorn Mountains, Wyoming

In the southern Bighorn Mountains two groups of mafic dikes occur within a terrain dominated by quartzofeldspathic gneiss. Un-metamorphosed dolerite dikes possess ophitic or subophitic texture and consist primarily of plagioclase (average composition, Ans 60 and augite (average Ca:Mg:Fe = 38:44:18). Metadolerite dikes in the same area consist of two distinctive facies. In each dike a granoblastic margin facies of clear plagioclase (average composition, An 38 ) and hornblende grades into an interior facies possessing relict ophitic or subophitic texture and consisting of clouded plagioclase (average composition, An 49 ) and granular augite (average Ca:Mg:Fe = 42:45:13). Many of the equant augite grains are peripherally replaced by hornblende. The metadolerites, similar in bulk chemical composition to the dolerites of the area, formed by recrystallization of comparable dolerites during the last regional metamorphism (low amphibolite facies) which affected the gneiss country rock. The dikes were metamorphosed under water-poor conditions which allowed only their margins to be completely recrystallized. Dike interiors underwent partial recrystallization accompanied by diffusion of iron from augite to relict plagioclase, causing clouding, and diffusion of calcium from plagioclase to form slightly more calcic augite and hornblende.

Tectonic Control of Devonian Reef Sedimentation, Alberta: ABSTRACT

Kaybob is one of a group of geologic reefs which underlie west-central Alberta and which comprise the Upper Devonian Swan Hills Formation of the Beaverhill Lake Group. Kaybob is a flat north-south elongate lens, 250 ft thick, 11 mi long, and 3 mi wide, built on the Slave Point Formation, a widespread platform carbonate. Detailed core study of sedimentary structure, texture, and constituents reveals that the carbonate sediments composing the can be grouped into 3 environmentally controlled facies groups: reef-slope, reef-margin, and shelf-lagoon facies. The reef-slope facies is a stromatoporoid-crinoid-brachiopod grainstone. The marginal deposits consist of a groundmass of massive stromatoporoid grainstone containing varied concentrations of different stromatoporoid growth forms (ranging from slender branching to large massive heads). The interior facies includes several rock types arranged in repeated vertical sequences. A basal unit of massive wackestone is overlain by combinations of massive, thin-bedded, and laminated mudstones, mudclast grainstones, and amphiporid carbonate conglomerate. Submarine and subaerial scoured surfaces are present within the upper units. The outer slope sediments intertongue with contemporaneous open-marine deposits seaward and with the reef-marginal facies on the inner side of the reef. This pattern changes laterally in vertical section, as the thickness of the changes from place to place. The upper half of the shows a marked westward displacement. Position of the various facies within the geologic mass, and comparison with Holocene carbonate sediments from several Caribbean localities, together provide a paleoenvironmental interpretation. The interior facies includes most of the sediment types which have been described from recent shallow shelf-lagoon environments. Sequential arrangements are the same and result from similar processes. The marginal realm includes many facies comparable with those observed in Holocene reef tracts, ranging from scattered coral growth on a sand bottom to coral constructed buttresses of ecologic reefs. Circumferential variations in geologic development, the westward displacement of the upper part, and a thick pile of open-marine deposits on the southwest are attributed to prevailing nor h winds during the Devonian. Tectonic control is exhibited at 4 levels. 1. The interior shelf-lagoon sequences are initiated by small-scale pulses of subsidence, perhaps complicated by eustatic sea-level changes. 2. Larger scale subsidence variations account for the thickening and thinning of the geologic body as a whole. 3. An orthogonal pattern of sharp elongate folds trending NE-SW and NW-SE is expressed clearly at the base of the reef. The folds are confined to the area of reefing, where they controlled initiation by forming mud mounds during upper Slave Point deposition. The pattern may reflect Slave Point block faulting. 4. This and others in the region, together with their associated carbonate-shelf deposits, fit into a well-expressed orthogonal tectonic pattern controlled by larger scale ba ement features. The basement features include a family of NE-SW-trending, relatively stable arches revolving about a NW-SE-trending major arch, the West Alberta ridge. They form part of the system of stable arches that provides the tectonic framework of the continent. End_of_Article - Last_Page 854------------