الفهرس 
LithologyOnly 14 pages are availabe for public view
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Abstract
The ,major part of the present study is the systematic account of Fayoum diatoms with their paleoecological significance and geographical distribution. It includes species and fdrms from 209 samples of surface sediments from 17 stations in 4 localities representating the widely extended ancient Qarun Lake. A total of 2’12 different diatom taxa belonging to 29 genera, 24 families, 8 orders and 2 subclasses were recorded. Of these 3 species and 11 varieties are considered to be new to the science.These species and varieties are : Gomphonema allami, Gomphonema bassiounii , Ni tzschia juxi, Gomphonema gracile var. demiaensis, Gomphonema parvulumvar. qarunensis, Cymbella ventricosa var.rhomboidea, Epithemia goeppertiana var.kom oshimensis, Epi themia reichel ti var. fayoumensis, Epi themia sorex var . noncapi ta ta , Epithemia zebra var. minuta, Epithemia zebra var. protracta, Rhopal odia rhopala var.aegyptiaca, ~itzschia amphibiavar.saghayensis,Surirella biseriata var. rayanensis. The following table elucidate a summary of the recognized genera and species in the present study. Composition of the diatom assemblage leads to the following paleoecological interpretations: 1) pH-spectrum: All the recorded diatom species in the Fayoum diatomites indicate that the diatomaceous sediments were deposited in alkaline water with pH value ranging from 7 to 8.5. This is refered to by the predominance of the alkaliphilous taxa ranging from 60% to 85% and represented by the greatest number of the recorded species, while the alkalibiontic species were found in small ratio between 15 to 30%, and were reported by small numbers of species; indifferent forms occurred in low values. 2) Halobian spectrum: Nearly all populations recorded in the Fayoum diatomites displayed a striking oligohalobous development, where the meioeuryhaline forms occurred in abundance and the major part of the recorded diatoms associated with common occurrence of mesoeuryhaline taxa and frequent occurrence of pleioeuryhaline species. Mesohalobous taxa was found in small numbers represented only by about 14 species such as Amphora commutata, Caloneis westii, Mastogloia braunii, M. elliptica, M. dansei, M. smithii, M. smithii var . amphicephala, Anomoeoneis sphaerophora, Nitzschia apiculata, N. hungarica, N. tryblionella, Surirella ovalis, 5. peisonis, Campylodiscus echeneis . The presence of these mesohalobous species in some examined samples associated with oligohalobous forms indicate that deposition of the diatomaceous sediments in the Fayoum depression occurred in alkaline fresh water of oligohalobian type with occasional slight salinity that may have reached up to 20%,.in some phases of lake development. This is also indicated by the appearance of some Ostracoda species that belong to brackish water type. 3) Temperature: Surface water temperatures during the time of deposition ranged from 15 to 20 Co; this is owing to the abundance of Melosira granulata, M. granulata var. angustissima,Cy dokella meneghiniana, and frequent to common occurrence of Cyclotella kiitzingiana which are usual.ly develop in warm water (15 to 20 CO,). 4) Eutrophication: The results of the investigation of the diatom assemblages show a distinct fluctuation in the troGhic level of the ancient lake from oligotrophic to mesotrophic-eutrophic to a fully eutrophic lake. This is estimated from the fluctuations in the abundance of the planktonic forms which are well developed in the eutrophic lakes, and pennate, epiphytic or benthonic taxa which are found in mesotrophic-eutrophic conditions with some forms developed in oligotrophic water. The changes in the composition. . of assemblages was refered to fresh water influx from the Nile into the old lake, which allowed oligotrophic assemblages to flourish; whereas the reduction of lake volume, which occurred several times due to the restricted fresh water influx and / or by excess evaporation caused the change into eutrophic fresh water stage with an abundance of the planktonic centrals forms. These changes in the trophic status have been estimated by using the quotient of Centrals : Pennales which reflected the content of nutrient matter in the environment. 5) Dissolved silicate concentration: The past fluctuations in the dissolved silicate concentration in the old lake was defined through the ratio between the maximum growth of S t e p h a n o d i s c u s and M e l o s i r a spp. According to KILHAM (19711, S t e p h a n o d i s c u s a s t r a e a has low silica requirements and its greatest. abundance occurred in eutrophic lake when the dissolved silicate concentration reaches a value below 1 mg/l; while M e l o s i r a g r a n u l a t a seems to be higher in silica requirements and becomes most abundant in eutrophic lake when the dissolved silicate concentration reaches values between 5 to 20 mg/l. 6) The saprobian spectrum for the Fayoum diatomaceous sediments indicates oligosaprobic to mesosaprobic conditions as assumed from the abundance of 21 oligosaprobic species associated with 15 diatom taxa characteristic of oligosaprobic-mesosaprobic and 32 species and forms characteristic for P- and &- mesosaprobic environments. Diatoms characteristic for polysaprobic were not encountered in the examined Fayoum -samples. 7) The current spectrum of the ancient lake was characterized by stagnant and running water as revealed by-the abundance of both limnobiontic and indiffere-nt taxa.