Browsing by Author "Baaske, Uwe Peter"

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Sequence stratigraphy, sedimentology and provenance of the Upper Cretaceous siliciclastic sediments of South Jordan
(2005) Baaske, Uwe Peter; Krawinkel, Hannelore (PD Dr.)
The aim of this study was to establish a sequence stratigraphic framework for the Upper Cretaceous (Albian to Santonian) sedimentary rocks of South Jordan. This was accomplished by detailed sedimentological studies and facies analysis. Furthermore the provenance and weathering history of the siliciclastic rocks was determined by using sedimentary petrography and bulk rock geochemistry. The study area is situated in the southern part of Jordan, about 60 km south of the city Ma'an, between the city Ras en Naqb in the West to Naqb Ataik in the East. Here a succession of Palaeozoic to Cretaceous sedimentary rocks is exposed along a NW-SE to E trending, about 90 km long escarpment. Along this escarpment fifteen vertical sections were logged in detail and samples where taken in the Cretaceous deposits. On the basis of these logs the facies analysis and sequence stratigraphic interpretation was carried out. Selected samples were used for petrographical and geochemical studies. Facies analysis showed that the Upper Cretaceous sediments were deposited in a terrestrial to marine setting. The examined sedimentary succession comprises ten environmental depositional facies, which can be grouped into four major facies associations. These four facies associations are (i) the shelf facies association, (ii) the shoreface facies association, (iii) the marginal marine facies association and (iv) the coastal plain facies association. The repeated interfingering of terrestrial and marine environments shows, that the examined sediments were deposited in a shallow marine siliciclastic to mixed siliciclastic-carbonate coastal system. The facies correlation of the logged sections reveals that the sedimentary succession can laterally be sub-divided into three major depositional systems: (i) a siliciclastic ramp, (ii) a mixed carbonate-siliciclastic ramp and (iii) an alluvial system. Vertically these depositional systems are arranged in repeating cyclothems. An ideal cyclothem consists of a retrograding siliciclastic ramp system, a mixed siliciclastic-carbonate ramp system, a prograding siliciclastic ramp system and is ended by an alluvial plain system. The shelf system was generally wave/storm dominated, whilst the prograding siliciclastic ramp system was supply dominated. The retrograding siliciclastic ramp, as well as the mixed siliciclastic-carbonate ramp system was accommodation-dominated. The coastal system can be divided into a regressive and a transgressive system. Under regressive sea-level conditions sand dominated braid deltas formed. Transgressive sea-level conditions resulted in a coastal system controlled by wave dominated estuaries and barrier/lagoon complexes. On the coastal plain anastomosing to meandering sand dominated river systems with wide overbank areas developed during transgressive times. During regressive sea-level conditions the nearshore river system changed to a more braided one that was directly linked to the braid deltas forming during these intervals. The sequence stratigraphic interpretation of the correlated sections shows that the studied sedimentary succession includes eight third-order sequences. The basal four sequences comprise the Cenomanian to Coniacian Ajlun Group, while the top four sequences are situated within the Coniacian to Santonian Belqa Group. Moreover sequences 1, 2 and 4 can each be further subdivided into two sequences of higher order. Basically all sequences include forced regressive deposits formed during the relative sea-level fall. Characteristic features of forced regressions, like for example sharp based shoreface deposits, are frequently present within the logged sections. Due to the significance of the regressive part of the relative sea-level for the studied sediment succession the sequence stratigraphic model introduced by Plint and Nummedal (2000) was used. This sequence stratigraphic model combines the complete sea-level fall in a new falling stage systems tract (FSST), which opposes LST, TST and HST. Comparison of the sequence stratigraphic interpretation with the major depositional systems shows that the depositional systems correspond very well with the four systems tracts of the applied sequence stratigraphic model. The alluvial plain system developed during the LST, the retrograding siliciclastic ramp system during the TST. The mixed carbonate-siliciclastic ramp system corresponds to the HST and the prograding siliciclastic ramp system is associated with the FSST. Relative sea-level changes and sediment supply were the main controlling parameters for the sequence architecture of the examined deposits. Since subsidence was low and no synsedimentary tectonic influenced the examined sediments, the relative sea-level changes are basically eustacy driven. Therefore the reconstructed sea-level curve resembles strongly global curves. The siliciclastic sediment supply is related to sea-level changes and associated with back stepping fluvial incision during the FSST and LST. Light mineral analysis shows that the Cretaceous sands and sandstones of South Jordan are basically mineralogical mature and can be classified as quartz arenites. Only the fine to medium grained sands and sandstones of the study area exhibit a slightly higher content of feldspar, classifying them as subarkoses. Heavy mineral analysis reveals a dominance of the ultrastable heavy minerals zircon, tourmaline and rutile. Especially zircon is predominant in the examined samples. With a mean ZTR-index of 97.9, the heavy mineral content reflects the mineralogical maturity of the sediments shown by the light mineral analysis. The studied sands and sandstones experienced a rather restricted spectrum of diagenetic processes. The compaction of the sediments was low to moderate, which is for example reflected by high IGV values. This and the lack of destructive grain contacts and pressure solution indicate that the sediments did not encounter deep burial or thick overburden. Concerning the cementation of the examined sediments, the sandstones display three major cementing phases. Sandstones cemented by clay minerals are only weakly lithified and display the highest intergranular porosities of all samples. Sandstones cemented by carbonate minerals (calcite and dolomite) or iron-oxides are well lithified and usually display only low values of intergranular porosities. The cementation with carbonate minerals is facies dependant and only occurs in marine sediments. Both, petrographic and geochemical provenance analysis point to the deposition of the Cretaceous sandstones of South Jordan within a passive continental margin setting. As source rock for the studied samples a continental block and/or a craton interior is most likely. Furthermore, a clear trend towards a recycled sedimentary source is obvious in the petrographical as well as in the geochemical studies. Especially the presence of resedimented quartz grains and the enrichment of zircon in the sediments support this. The presented study shows that the Arabo-Nubian-Shield and its Palaeozoic sedimentary cover are the source area for the Cretaceous siliciclastic sediments of South Jordan. The examined sediments follow normal weathering trends in the weathering diagrams. The corresponding CIA values point to a moderate weathering for most samples. Only the sandstone and some claystone samples have high CIA values typical of recycled sediments.