Middle – Late Jurassic (Late Bathonian – Tithonian) Palynomorphs

INTRODUCTION Jurassic palynomorph assemblages have been recovered in numerous wells in northeast Libya. Jurassic rocks reflect changing sedimentary environments which have greatly influenced the composition of the palynological assemblages. In the northernmost area, Jurassic sediments unconformably overlie the Palaeozoic or Triassic and show a mixed marine and continental influence. In the northeastern part of the area thicker and deeper water marine sediments are known, while shallow marine sediments overlie the platform facies immediately to the south. Pollen and miospores are fairly well preserved and are dominant in most samples. Dinoflagellate cysts are richly represented especially in the deposits of the north and northeast. Most samples contain abundant variably sorted cuticular debris and structured wood fragments. This significant influx of terrestrial debris together with the associated palynomorph assemblages indicate deposition in a shallow marine environment in close proximity to the shoreline for most of the Jurassic deposits in the northern area. In the central and southern region, sandstone, silt stone and red shale deposited in non-marine fluvial to lacustrine or lagoonal environment, unconformably overlie the metamorphic or igneous Precambrian Basement. These sediments show a general lack of well preserved palynomorphs. Miospores, though present are generally long ranging and terrestially derived detrital kerogen dominate the assemblages. MICROFLORAL SUCCESSION Miospore assemblages present in most of the samples investigated are dominated by small gymnosperm pollen. Classopollis spp., Exesipollenites spp., Sphaeripollenites spp., and nonaperturate pollen assigned to Araucariacites spp. Saccate pollen assigned to Concentrisporites spp., Perinopollenites spp., Callialasporites spp., and Inaperturopollenites spp., are often common . . .


INTRODUCTION
Jurassic palynomorph assemblages have been recovered in numerous wells in northeast Libya. Jurassic rocks reflect changing sedimentary environments which have greatly influenced the composition of the palynological assemblages.
In the northernmost area, Jurassic sediments unconformably overlie the Palaeozoic or Triassic and show a mixed marine and continental influence. In the northeastern part of the area thicker and deeper water marine sediments are known, while shallow marine sediments overlie the platform facies immediately to the south. Pollen and miospores are fairly well preserved and are dominant in most samples. Dinoflagellate cysts are richly represented especially in the deposits of the north and northeast. Most samples contain abundant variably sorted cuticular debris and structured wood fragments. This significant influx of terrestrial debris together with the associated palynomorph assemblages indicate deposition in a shallow marine environment in close proximity to the shoreline for most of the Jurassic deposits in the northern area.
In the central and southern region, sandstone, silt stone and red shale deposited in non-marine fluvial to lacustrine or lagoonal environment, unconformably overlie the metamorphic or igneous Precambrian Basement. These sediments show a general lack of well preserved palynomorphs. Miospores, though present are generally long ranging and terrestially derived detrital kerogen dominate the assemblages.

MICROFLORAL SUCCESSION
Miospore assemblages present in most of the samples investigated are dominated by small gymnosperm pollen. Classopollis spp., Exesipollenites spp., Sphaeripollenites spp., and nonaperturate pollen assigned to Araucariacites spp. Saccate pollen assigned to Concentrisporites spp., Perinopollenites spp., Callialasporites spp., and lnaperturopollenites spp., are often common components of the assemblages. Spores are mainly represented by smooth, trilete forms. Sculptured forms are rare but in cutting samples they may be present as a result of contamination by caving.
The miospores present in this interval are long ranging and extend throughout the Jurassic sequence. The variable distribution observed within the sequence has been attributed to sedimentary control, mainly arising from the effect of transport and sorting of palynomorphs from the land vegetation area to the site of deposition.
The illustrated assemblages are similar to those reported by Reyre (1973) from Sahara who found that bisaccate pollen were common only up to zone 12B, and are replaced in Liassic and younger Mesozoic by the group of "large pollen". This group is again reduced during the later part of the Callovian and Late Jurassic. Our observations indicate a similar development in Northeast Libya. Based on assemblage characters and individual spore and pollen taxa, correlation of the Libyan assemblages can be confidently established with palynozones 5a and b of Reyre (1973). Thus, a Middle to Late Jurassic age is suggested for the Libyan miospore assemblages. No diagnostic miospores of Early Jurassic age were found in the samples examined. Classopollis is represented by sculptured and "smooth" forms that require study by the scanning electron microscope in order to ascertain their stratigraphical significance.
The Libyan assemblages show significant differences from those of European Jurassic deposits. Bisaccates are absent to rare in the Libyan assemblages, but are generally very common in Middle and Late Jurassic assemblages from Northern latitudes. Similarly, Araucariacites, Classopollis and Sphaeripollenites together with Callialasporites and Inaperturopollenites generally abundant in the Libyan assemblage, are relatively less abundant in the northern assemblages. There are thus strong similarities between the miospore assemblages of the Sahara and northeast Libya and marked differences from the European and Arctic microfloral assemblages as reported by Norris ( 1 970 Dinoflagellate cyst assemblages of Late Callovian-Early Oxfordian age are characterised by the appearance of Wanea digitata, Energlynia acollaris, Cribroperidinium granulatum, Korystocysta pachyderma and Gonyaulacysta scarburghensis. Energlynia acollaris is common in some samples and also occurs rarely in the older assemblage. Ellipsoidictyum spp., Gonyaulacysta filapicata, and several morphotypes of Sentusidinium disappear in this assemblage. The youngest assemblage is characterised by the appearance of palynomorphs of Late Kimmeridgian to Tithonian age, and is dominated by Millioudodinium globatum, and a few typical Late Jurassic species, such as Muderongia sp. A Davey, Ctenidodinium panneum, Leptodinium cf. aceras (Eisenack, 1958) Sensu Gitmez & Sarjeant 1972 and Lithodinia cf. jurassica Eisenack, 1935Sensu Ioannides et. al., 1976. Korystocysta kettonensis which is present in this assemblage may be reworked from the older assemblage.
In summary, the majority of the samples analysed in the northern and northeastern region contain Bathonian to Early Oxfordian dinoflagellate cysts. A limited number of wells (Al-36, AI-NC92, C1-2 and El-2) contain a sparse Late Jurassic microflora. Many typical Oxfordian and Early Kimmeridgian dinoflagellate cyst species known in the boreal areas were not encountered. This might suggest a possible microfloral provincialism but this suggestion must await the results of further studies, currently in progress.
In the south central and southern area, where deposition of sandstone, siltstone and red shale took place in a fluvial, lacustrine or lagoonal environment, probably under dry climatic conditions, a general lack of well preserved palynomorphs is evident. The most commonly encountered species include Cerebropollenites mesozoicus, Concavisporites sp., Classopollis spp., Araucariacites sp. and Deltoidospora sp.
Many of the forms encountered also occur in the overlying sandstone of Early Cretaceous age (see later contribution). Sediments of Jurassic age are frequently difficult to distinguish from those of Early Cretaceous             age, and are therefore collectively referred to as the Sarir or Nubian Sandstone of Late JurassiclEarly Cretaceous age in the study area. With the exception of a very few wells, where definite Neocomian-Barremian palynomorphs have been encountered, there is at present no palynological evidence to allow the separation of Jurassic and Early Cretaceous sequences in the south-central and southern areas.