Early Senonian Research Articles

BIOSTRATIGRAPHIC CHARACTERIZATION OF THE CRETACEOUS DEPOSITS OF THE SAN-PEDRO MARGIN OF DRILL HOLES K1 AND K2 OF BLOCK KL: PALEOENVIRONMENT IMPLICATION

The KL block studies was carried out the eastern part of the San Pedro margin, it has an area of 2034 km2 with a water depth varying from 500 to 2750 m with two probings (K1 and K2).The objective of this work is to carry out a biostratigraphic and paleoenvironmental study based on the associations of planktonic and palynomorphicmicrofauna in the formations of the KL block boreholes. From a lithological point of view, the base of the boreholes generally comprises alternating limestone and argillite, very fine to fine grained quartz sandstone. Its upper part is overlain by claystoneinterbedded with limestone, silstone and siliceous cemented sandstone and alternating claystone, medium to coarse grained sand and siliceous cemented quartz sandstone. The Albian is determined by the presence of the species Ticinellamadecassiana. The Cenomanian is identified by the micropalaeontological assemblages composed of Globigerinelloides spp., Guembelitria spp., Hedbergella spp., Hedbergelladelrioensis, Globigerinelloides bentonensisandLoeblichella cf. hessi. The Turonian is based mainly on the species Hedbergellaplanispira, Heterohelixmoremani and Whiteinella archaeocretacea. The Early Senonian is characterized by associations of species (Hedbergellasp, Hedbergella cf. delrioensis, Buliminacrassa and Whiteinella baltica) and palynomorphs (Proteaciditestienabaensis, Odontochitinacostata, Odontochitinaporiferaand Tricolpites sp). The roof of the Campanian is known by the association of the palynomorph (Trichodinium castanae) and the microfossil (Gaudryina cretacea) The Maastrichtian is highlighted by the associations composed of species Rzehakina epigona fissistomata, Rzehakina minima, Plectina lenis, Reophax duplex, Reophax pilulifera, Reophax globosus, Gaudryina pyramidata and Afrobolivina afra) and palynomorphic species (Andalusiella gabonensis, Cerodinium granulostriatum and Palaeocystodinium australinium). All the micropalaeontological data coupled with those of the microfaunas make it possible to envisage a depositional environment of the internal platform type with continental influence on an external platform.

Early Senonian climate reconstruction for southwestern Siberia on the basis of paleobotanical data

On the basis of study of fossil plants, the climate in southwestern Siberia in the Early Senonian was reconstructed to be warm temperate humid with the mean annual temperature of about 13°C. The CLAMP analysis (Climate Leaf Analysis Multivariate Program) was applied for the first time to Cretaceous floras of Western Siberia. The comparison results of climatic variables estimated show that the climate in Western Siberia in the Early Senonian was more humid and warmer than in Northeast Asia.

New paleomagnetic results from imbricated Adria: Ist Island and related areas

For the purpose of this work samples for palaeomagnetic analysis were taken from Upper Cenomanian and Lower Senonian shallow water limestones, as well as from Senonian pelagic limestones from both Ist and the surrounding islands. This area belongs to Imbricated Adria, which is characterized by gently folded and faulted strata with a Dinaridic (NW–SE) trend. An exception is Premuda island where the beds are strongly folded and are subvertical. A total of 96 samples were drilled from 10 localities distributed between eight islands. The samples were then subjected to standard palaeomagnetic laboratory analysis and statistical evaluation. Eventually, six localities yielded statistically well-defined palaeomagnetic directions, which were shown pre date the folding in age. The overall mean palaeomagnetic direction obtained for the study area, characterizing the Cenomanian–Early Senonian time period had a Declination of 334°, Inclination=+46°, with statistical parameters k=188, a 95 =4.9°, defining a palaeomagnetic pole at l (N)=63°, f (E)=254°, d p=4.0°, d m=6.2°. This was compared with palaeomagnetic directions obtained for rocks of similar ages from stable Istria and the Kvarner islands. As the three palaeomagnetic directions are statistically identical, we conclude that there was no significant relative movement between the three areas after the Early Senonian. The palaeomagnetic declination for the study area, which characterizes the post-Early Senonian rotation of the Adriatic microplate, is the same as the declination for the Pannonian–Pontian age group from the South Pannonian basin. As the palaeomagnetic signals in both cases are primary, the results of the present paper not only support the conclusion that the rotating Adriatic microplate triggered rotations in the South Pannonian basin, but also suggest that the Adriatic platform did not change its orientation between the late Cretaceous and the Early Pontian.

Early Senonian radiolarian microfauna and biostratigraphy from the Western Vardar Zone (Western Serbia)

Early Senonian radiolarian microfauna and biostratigraphy from the Western Vardar Zone (Western Serbia)The studied deposits represent the sedimentary cover of ophiolitic mélange of the Western Vardar Zone Belt. An association of sediments that correspond to a primary pyroclastic material occurs in the Upper Cretaceous carbonate sediments near the village of Struganik (Western Serbia). This is an interlayer within mainly carbonate sediments represented by limestone, clayey limestone and marlstone. It is made of the following succession: a lamina made of crystalline quartz, sanidine, plagioclase and biotite and a layer of clay. The clay is of the smectite type, highly crystalline. The age of the radiolarian assemblage from the clay layer is assigned to Early Senonian, based on the co-occurrence of radiolarian taxa:Dictyomitra formosaSquinabol,Dictyomitra koslovaeForeman,Dictyomitra torquataForeman,Alieviumsp. cf.A. superbum(Squinabol) andPseudoaulophacussp. cf.P. venadoensisPessagno. The pyroclastic material was brought into the water environment by a cloud that was formed after an explosive eruption whose exact location cannot be determined at the moment. According to geological data, there are no indications of volcanic activity before the Late Cretaceous in the wider studied area. Marine sedimentation continued after gravitational differentiation of pyroclastic material. The results of petrological and sedimentological investigations reveal that Struganik Limestone originated in a deep-water environment and that the sedimentation area was on the continental slope.

The first Microcharon (Crustacea, Isopoda, Microparasellidae) from the Moroccan North Saharan Platform. Phylogeny, origin and palaeobiogeography

The interstitial stygobites of the genus Microcharon (Crustacea, Isopoda, Microparasellidae) are highly diversified in Morocco, especially in the High Atlas. A new species from the North Saharan platform is described. Microcharon oubrahimae n. sp. is characterized by the original morphology of the first male pleopod which exhibits a concave inner margin of the distal part and a subdistal position of the armature. From a phylogenetic point of view, M. oubrahimae does not belong to the lineage which includes the Moroccan Atlasian species. In contrast, it belongs to the eastern- Mediterranean group of species. It is related to the species of the groupM. orghidani -M. bureschi -M. phlegetonis from Romania and Bulgaria. The two-step model of colonization and evolution provides an understanding of the origin and evolutionary history of this stygobiont. M. oubrahimae derived from marine ancestors that lived in the littoral interstitial waters of the marine gulfs which covered the Errachidia-Boudnib-Erfoud basin within the pre-African trench during the Turonian or more likely Early Senonian. These marine ancestors might have settled in fresh groundwater during the regressive phases of the Turonian embayment or more likely of the brief Coniacian-Santonian gulf.

Tectonique synsedimentaire d'age cretace superieur en Tunisie nord orientale; blocs bascules et reorganisation des aires de subsidence

Abstract Structural analysis of late Cretaceous sequences from the northeastern Tunisian Atlas, led to conclude on an active basin floor instability. Regional tectonics resulted in tilted blocks with a subsidence reorganization, since the Campanian time. These structural movements are controlled both by N140 and N100-120 trending faults. The Turonian-Coniacian and Santonian sequences display lateral thickness and facies variation, due to tectonic activity at that time. During Campanian-Maastrichtian, a reorganization of the main subsidence areas occurred, the early Senonian basins, have been sealed and closed and new half graben basins developed on area which constituted previously palaeohigh structures. These syndepositional deformations are characterized by frequent slumps, synsedimentary tilting materials, sealed normal faults and progressive low angle unconformities. These tilted blocks combined to a subsidence axis migration were induced by a NE-SW trending extensional regime. This extension which affects the Tunisian margin during the Upper Cretaceous, is related to the Tethyan and Mesogean rifting phase which resulted from the combined movements of the African and European plates.

Genesis of Late Cretaceous toe-of-slope breccias in the Bakony Mts, Hungary

Abstract Toe-of-slope megabreccia deposits of an Upper Cretaceous rudist platform were studied in the Bakony Mountains. During the Turonian–early Senonian tectogenesis an articulated basin came into being in the area of the Bakony; depressions and highs were formed roughly parallel with the structural strike of the mountains. Inundation of the highs led to the evolution of carbonate platforms. The investigated platform shows an asymmetric architecture. In contrast to its gentle northern slope a steep erosional slope bounded the platform to the south with large, low-angle aprons, the site of deposition of megabreccias. The breccia accumulation occurred within a single 3rd-order cycle commencing in the early highstand and reaching its climax in the late highstand.

Sedimentological evolution of the Cretaceous carbonate platform of Chiapas (Mexico)

The Cretaceous epicontinental carbonate platform of Chiapas (south-east of Mexico) extends along a 200 km NW–SE narrow strip, north of the Sierra Madre basement, from Ocozocoautla to Comitan. In the western and central domain, three stratigraphic sections of the Sierra Madre Formation (late Aptian to early Senonian) display well exposed facies sequences enabling broad facies correlations about: (1) The successive transgressive–regressive stages, (2) the different subsidence rates controlling the outer to inner platform environmental evolution, (3) the conditions of tectonically controlled partial platform drowning during Campanian–Maastrichtian. Three other sections through the eastern Maastrichtian carbonate platform area record the changes from limestone to dolomite during the Angostura Maastrichtian platform stage. This evolution of thickness and facies in the occidental domain of Piedra Parada and in the central domain of Guadalupe Victoria and Julian Grajales illustrates the settlement process of the carbonate platform and the general decreasing of the thickness of the Sierra Madre Formation from west to east. The eastern platform domain (Comitan) crops out extensively and enables new correlations along a south–north transect. The Sierra Madre Formation and Angostura Formation documents continuous carbonate platform sedimentation with foraminifers, rudists and dasycladacean algae during Campanian and Maastrichtian. These sections permit palaeogeographical comparisons of depositional conditions of the Mexican margin of the Maya block.

Facies analysis of an Upper Cretaceous high-energy rudist-dominated carbonate ramp (Matese Mountains, central-southern Italy): subtidal and peritidal cycles

Upper Cretaceous shallow-water limestones rich in rudists have been studied in the Matese Mountains. They show a preponderance of skeletal components (molluscs and benthic foraminifers) and a lack of non-skeletal grains. The most prominent sediments of the studied stratigraphic interval are rudist-dominated fine- to coarse-grained lithologies. Most of the Senonian successions are characterized by massive neritic limestones rich in molluscs (mostly rudists and subordinate gastropods) and a few corals. The sediments were generated in situ on shelves where rudist bivalves were the primary sediment producers. These sediments were actively moved by storms, waves and swells. The finer fractions were probably winnowed out and deposited in deeper water. Well-bedded dolomitized mudstones/wackestones, microbial laminites and benthic foraminifer wackestones characterize some successions where they rhythmically alternate and testify to deposition in tidal-flat environments. The lithofacies, faunal and taphonomic characters and the sedimentary structures are typical of a ramp-like open shelf with local peritidal deposition. The vertical lithofacies organization consists of numerous decimetre- to metre-scale shallowing-upward subtidal and peritidal cycles. A complete transition from peritidal to subtidal cycles has been recognized and documented. The peritidal cycles are usually bounded by desiccation surfaces and/or dolomitized crusts followed by shallow-subtidal to peritidal facies of the next cycle. The subtidal cycles are typified by rudist floatstones passing to large-scale cross-bedded bioclastic packstones/grainstones. Current and storm structures are distinctive features; in shallower-water environments the effects of submarine erosion prevail; hardgrounds commonly developed and the resulting cycles are usually truncated. The formation of the peritidal cycles appears to have been controlled by accommodation space in response to fluctuations in relative sea level. The formation of the subtidal cycles was determined by sedimentation rates in response to fluctuations in the zones of fair-weather and storm-wave reworking, in turn controlled by relative sea level. This indicates that sediment production and supply were important controlling factors in the cycle formation during the early Senonian stratigraphic interval. The recognition of similar depositional styles in other early Senonian successions cropping out in the southern Apennines suggests that this type of sedimentation is a recurring attribute of the Upper Cretaceous open shelves.

Protopterus nigeriensis nov. sp., l'un des plusanciens protoptères — DIPNOI — (In Beceten, Sénonien du Niger)

The In Beceten locality (Early Senonian) in the southern Niger has yielded two dipnoans Neoceratodusafricanus and Protopterus nigeriensis nov. sp., this near species is probably closely related to Lavocatodus protopteroides.

On Asian ornithopods (Dinosauria: Ornithischia). 2. Arstanosaurus akkurganensis Shilin and Suslov, 1982

The hadrosaurid dinosaur Arstanosaurus akkurganensis was described on the basis of the posterior end of a left maxilla, an associated distal end of a femur and an unreported isolated tooth, all of which were collected from the Akkurgan locality. East of the Aral Sea in central Kazakhstan. Redescription of the type material reveals that the material was inaccurately described, appears to lack specifically diagnostic characters and, although it can be assigned tentatively to the hadrosaurid subfamily Lambeosaurinae on the basis of the form of the caudodorsal margin of the maxilla, should for taxonomic purposes be regarded as a nomen dubium . Arstanosaurus cannot be assigned to any known hadrosaurid taxon from Kazakhstan with any confidence at present partly because of a lack of comparative material, and difficulties of comparative dating across what appear to be very similar localities (e.g. Shakh-Shakh). Broad anatomical similarities can be drawn with the Chinese genus Bactrosaurus . Additional hadrosaurid material collected from Baynshin Tsav, Mongolia, previously attributed to the genus Arstanosaurus , represents two new ornithopod taxa assignable to the subfamilies Hadrosaurinae and Lambeosaurinae. General comparison between the forms collected from Akkurgan, Iren Nor (China) and Baynshin Tsav supports the contention that all three localities may be dated as Early Senonian (Coniacian-Santonian) in age.

Chlorozoan versus foramol lithofacies in Upper Cretaceous rudist limestones

The study of Upper Cretaceous limestones of the Nurra region (northwestern Sardinia, Italy) that crop out with open shelf, rudist-rich deposits has shown that during the early Senonian (Coniacian-Santonian interval) sediments characterized by assemblages dominated by molluscs, with variable amounts of red algae, bryozoa and echinoids, replaced sediments including hermatypic corals, green algae and variable amounts of non-skeletal grains. These latter components were largely dominant in the previous carbonate-shelf contexts of the Nurra during Jurassic-early Cretaceous times, becoming less numerous until they disappeared in the transgressive deposits that followed the tectonically induced middle Cretaceous emersion phase. A rhodalgal association (typical of anomalous tropical areas or transitional ones) has been recognized in the replacing limestones that may be more generically considered foramol-type deposits. This seems to document conditions of stress in the waters impinging on the Nurra carbonate platform during the Coniacian-Santonian transition. The observation that, in the late Cretaceous, a similar evolutionary trend seems to have characterized many shallow neritic successions, from many different areas, leads us to hypothesize more general causes of stress (e.g. induced upwellings on the marginal sectors of the shelves, increased oceanic overturns and hyperproductivity due to terrestrial runoff on post-emersive transgressed substrata). Peculiar oceanic conditions were able to create depositional contexts that were considerably changed with respect to the previous ones, such that the more opportunistic forms of the foramol assemblages flourished. The change in dominant assemblages resulted in a different organization of the sediment as a response to the different producing communities. The resulting temperate-type carbonate shelves were characterized by a low growth potentiality essentially due to a higher dispersion rate and this could have facilitated their drowning.

Ptolemaïs basin: The tectonic origin of a Senonian marine basin underneath the southeastern Mediterranean Sea

The large Litani anticline, that was recently discovered in the SE Mediterranean basin, sheds new light on the regional tectonic history of the Levant in the late Cretaceous. The anticline is buried under 5 km of undeformed sedimentary series, its apex was leveled by erosion, and its folding can be dated to the early Senonian. The seismic reflection data suggest that shortly after the structural development of the anticline in a shallow marine environment and under compressional stresses, the regional tectonic regime turned extensional, and the anticline subsided into the open marine environment of Ptolemaïs basin. That subsidence can be dated to the Senonian as well. It is suggested that the Litani anticline is a part of the Syrian Arc, and its deformation and subsequence subsidence reflect the complex tectonic regime of southern Anatolia, where the continental collision with Arabia and the subduction of the Neo-Tethys oceanic crust took place concurrently. The Litani anticline and the Ptolemaïs basin thus show that contrasting tectonic regimes and their structural derivatives could evolve at a very fast rate.

Sedimentologic and paleotectonic evolution of carbonate platforms on a segmented continental margin: example of the African Tethyan margin during Turonian and Early Senonian times

During Turonian and early Senonian times, there were extensive shallow-water carbonate platforms on the northern margin of the African craton (Eastern Algeria, Central Tunisia and adjacent offshore areas), at the edge of the Tethys ocean. The development of these carbonate platforms (mid-Turonian to late Coniacian) was especially controlled by the segmentation of the African (Saharan) margin by extensional fault systems, as evidenced by block-faulting, volcanic activity and halokinetic movements. From the mid-Turonian to early Coniacian times, the carbonate platforms underwent an incipient drowning, indicated both by their sedimentologic evolution (rimmed-shelves to carbonate ramps) and the landward shift of facies belts. A partial drowning of carbonate platforms during late Turonian times was probably induced by climatic and/or oceanographic changes. The late Coniacian is the period of continuation and further extension of deepening events with the widespread development of deep ramp facies. At the Coniacian-Santonian boundary, the sinking of the Saharan margin induced an abrupt increase of subsidence rates and the complete drowning of carbonate platforms, covered by deep-water argillaceous facies. This renewal of tectonic activity at the Coniacian-Santonian boundary is recorded in many areas in North Africa (“Intra-Senonian phase”).

Eastern Iberides Cretaceous Basin (Spain) Compared to the Western Pyrenean Basin (France): Genesis, Geodynamic Evolution: ABSTRACT

The Cretaceous eastern Iberides basin was created by the breakup of the Jurassic platform, which emerged at the end of the Valanginian (Neocimmerian movements). Up to Albian time, two separate basins (the northern Maestrazgo and the southern Penagolosa) can be recognized. They correspond to half-graben oriented Azimuth 140, where the sedimentary series was controlled by both eustatism (sequential organization) and local tectonics (depocenter migration toward the southwest). Halokinetic structures are also related to this extensional phase since they are expressed in the Penagolosa zone. During Albian time, tectonic movements decreased and clastics, followed by carbonate deposits, covered the whole area. Then during the Late Cretaceous, carbonates, whose framework is mainly controlled by eustatism, filled the large, newly individualized basin. However, the early Senonian showed change from Mesogean to Atlantic polarity, indicating a different regional tectonic stress (north-south extension followed by north-south compression). Paleogene to Neogene compression had minor effect in the Iberides. Thus synsedimentary structures can be well studied in a preorogenic setting. The western Pyrenean basin presented a similar evolution during the Early Cretaceous. However, during the Albian a major change was represented by the opening of the Gascogne Gulf. Consequently, a highly complex subsiding basin was generated duringmore » the extension process. Furthermore, the Pyrenean structures resulting from north-south compression were strongly influenced by the Mesozoic inherited framework, which has been analyzed using Iberian data.« less

Lithostratigraphy and biostratigraphy of the lewes and seaford chalks: A link across the Anglo-Paris basin at the Turonian-Senonian boundary

The positioning of the Turonian/Senonian boundary has been highly discussed since the introduction of the Senonian stage by d'Orbigny. In England the Early Senonian is conventionally represented by white chalks at the base of the Micraster cortestudinarium zone which to some has meant the top surface on the Top Rock or Navigation Hardgrounds. New macro- and microfaunal data on the Sussex White Chalk of Southern England, while providing a suitable framework for the comparison between the microfaunal zonation of the Paris basin chalks and macro- and microfaunal zonations commonly used in England, emphasize the positioning of the Turonian/Senonian boundary in the Anglo-Paris basin. This boundary falls within the interval Chalk Rock/Top Rock and is outlined by distinct changes in Micraster lineage (appearance of M. normanniae) and in benthic foraminiferal associations.

Comparaison entre les plates-formes du Crétacé supérieur de Provence et de Sardaigne

The development of a carbonate platform environment occurred later in Sardinia than in Provence. The platform regime was connected with positive eustasy in the Early Senonian. As in Provence, this regime followed emergence and folding of W Sardinia, correlated with movements deriving from the Austrian phase. Reefal elements (rudists and benthic foraminifers) provide a global backdrop for the palaeobiogeographical affinities between Sardinia and the Pyreno-Provençal domain. Although eastern influences can be discerned, they are slight and point to the existence of an oceanographical barrier between the Sardinian platform and that of the Apulian plate. Platform facies are well developed in SW and NE Sardinia, while basinal facies dominate to the east. Lithological considerations point to the rudistid reefs of Sardinia having developed in a more open marine environment, relatively distant from the land, which is not the case for Provence. A reconstruction of the position of the Corsican-Sardinian block relative to Provence in the Late Cretaceous is made on the basis of the stratigraphical and palaeobiogeographical data available. This reconstruction indicates possible relationships of the platforms of the western Mediterranean with the land-areas and the zones of the basin.

Paleomagnetism, K-Ar dating and tectonic interpretation of Upper Cretaceous and Cenozoic volcanic rocks of the Chatham Islands, New Zealand

Abstract New K-Ar age determinations on basaltic volcanic sequences in the Chatham Islands, 900 km east of South Island, New Zealand, combined with additional field and paleontologic studies, have led to a stratigraphic revision and the recognition of three distinct volcanic episodes—Late Cretaceous (70–85 m.y.), Eocene-early Oligocene (35–53 m.y.), and late Miocene-Pliocene (2–6m.y.). Paleontology, palynology, and K-Ar dating place the earliest alkali basalt-trachyte volcanism of the Kahuitara Tuff and Southern Volcanics in the early Senonian, at the time of separation of the Chatham Rise-Campbell Plateau block from West Antarctica. The Southern Volcanics were formerly considered to be Eocene and the Kahuitara Tuff was included in the Waihere Bay Group (Albian). The Waihere Bay Group (s.s.) of Albian-Cenomanian age comprises mainly estuarine to deltaic sediments, derived from the Chatham Rise and/or West Antarctica, prior to continental separation. The Eocene and Pliocene volcanics comprise limburgitic p...

A red-bed formation in the South Bohemian Basins, Czechoslovakia

Tectonic activity, climatic changes and associated weathering cooperated during the formation of Upper Cretaceous red beds of the South Bohemian Basins known as the Klikov Formation. The sediments are clastic and comprise: (1) light-grey or yellow conglomeratic, course-to-medium sandstone beds; (2) generally finer red beds; and (3) grey beds. They alternate in asymmetrical cycles, in succession fining upwards. Small- and large-scale cyclothems were recognized. The large-scale cyclothems are closely connected with paleogeographical changes and tectonic evolution of the South Bohemian Basins in Early Senonian time. In regard to rapid vertical and lateral lithological variations, the common presence of graded bedding, poor sorting and lack of fossils, the sediments are comparable to the fluviatile facies of molasse, known as “flysch in molasse”. They were deposited from highly fluctuating currents and periodically overloaded streams in an alluvial and/or lacustrine environment, suffering continuously from tectonic unrest emanating from the Alpine—Carpathian geosyncline. Differences between red and grey sediments are related mostly to the primary distribution and preservation of organic matter and to some extent to the redistribution of iron during early diagenetic changes (e.g. red stains). Ancient weathered gneiss and granite profiles preserved beneath the sediments show development of either two (on granite) or three (on gneiss) horizons differing in degree of weathering and distribution of red colour within the profiles. The trend of weathering documented by chemical analyses and presence of kaolinite as the end-product points to the red podzolic soil formation process.

Late Cretaceous Nappes in Oman Mountains and Their Geologic Evolution

The rocks of the central Oman Mountains can be divided into five main units. These comprise, from bottom to top: a pre-Permian basement that stratigraphically is overlain by mostly shallow-marine carbonate rocks ranging in age from Middle Permian to Late Cretaceous (Senonian). Above this, several thin and imbricated nappes, together with several large (mountain-size) exotics, form the allochthonous Hawasina unit. The age range of the strata within each of these nappes covers a part of the time span from Permian to Cenomanian. The sediments vary from the Permian and Triassic shallow-marine limestones of the exotics that were deposited over a substrate of pillow lavas, to limestone turbidites and cherts which locally are associated with minor amounts of basaltic pillow avas. In the northern mountains the Hawasina tectonically overlies Permian to Cenomanian sequences of reefal limestones, slump conglomerates, turbidites, and calcareous mudstone of the Sumeini Group. The Hawasina is overlain by the Semail nappe, an enormous mass of ophiolites. Minor amounts of pelagic sediment dated as Cenomanian and early Senonian are associated with the submarine lavas of the ophiolite suite. Both the Hawasina and the Semail are overlain unconformably by Maestrichtian and early Tertiary sedimentary rocks. From the sedimentary and structural complexities of this mountain range, the following simplified interpretations have been made. 1. The autochthonous Permian to Senonian shallow-marine carbonate sediments were deposited on the continental margin of Arabia. 2. The Sumeini Group was deposited on the northeastern edge and continental slope of Arabia. 3. The age range of each of the Hawasina sedimentary sequences is based partly on the recognition that many of the microfossil assemblages from turbidite sequences were derived penecontemporaneously (rather than reworked into a younger sequence) and form part of a normal stratigraphic succession. 4. The sedimentary rocks of the Hawasina nappes were deposited northeast of the present Oman Mountains between the Permian and the Cenomanian. 5. The rocks that now constitute the Semail nappe were generated in an upwelling part of the earth's mantle (peridotites and gabbros) and on the ocean floor (lavas). The upwelling is thought to have been associated with ocean-floor spreading and to have given rise to an oceanic ridge which formed the distal limit to that part of the basin in which the Hawasina sediments were deposited. 6. During the Late Cretaceous (late Campanian) the Sumeini and Hawasina sedimentary rocks and the Semail ophiolites were emplaced tectonically into their present location on the outer part of the Arabian continental margin. 7. After emplacement, the Hawasina and Semail nappes were covered unconformably by shallow-marine Maestrichtian and early Tertiary limestones. 8. The outer part of the Arabian continental margin (together with its cover of nappes and younger marine limestones) was uplifted during the Oligocene and early Miocene to form the present Oman Mountain range.