Palynological investigation of terrestrial palynoflora of the Glen Rose Formation, Texas and its implications for dating other Lower Cretaceous sequences

The Lower Cretaceous sediments of Texas have great importance in being transitional between terrestrial and marine environments. The Glen Rose Formation of latest Aptian to late early Albian age is an important example of this situation showing both terrestrial palynological assemblages (spores and pollen) and marine fossils (ammonites, foraminifers, etc.). The Glen Rose was deposited in a transgressive shallow marine environment and is well dated by marine ammonites. It forms the upper part of the Trinity Group and underlies the Fredericksburg Group of middle and late Albian age. There are important published works on palynology of Fredericksburg and overlying Washita Groups in Texas and Oklahoma. There are also some published palynological studies on the sediments possibly equivalent to the Glen Rose Formation in Arkansas. However, there is lack of information on the palynology of the Glen Rose except for one abstract and two unpublished theses.;Palynological study of the formation is essential for studying the appearance and diversification of early angiosperms. In this project, a palynological characterization of this formation is constructed, which may also provide a spore-pollen reference for correlation of continental deposits (e.g. Potomac Group of the Atlantic Coastal Plain). The Potomac Group forms the oldest unit of the Middle Atlantic Coastal Plain and was deposited in a continental environment. Because of the absence of marine fossil data the Potomac sequence is dated by only palynological correlations. Brenner (1963) established a zonation (Zone I and II including subzones II-A and II-B) in a comprehensive study of spores and pollen of the Potomac Group. There is a hypothesis of a possible disconformity between the top of Zone I and the bottom of Zone II due to the sudden appearance of many species at the bottom of Zone II. Recent studies from Portuguese sections of late Barremian to middle Albian age showed great consistency with this hypothesis. Finding new evidence on the extent of this break is essential since it may cause a shift on the age of the base of Zone II. Results of this project showed the occurrence of striate tricolpate pollen in the Glen Rose Formation, apparently older than in the Zone II of the Potomac Group consistent with the results of the sections from Portugal. In addition, the angiosperm assemblage of the Glen Rose Formation showed greater diversity of monosulcate grains than tricolpate grains, which is opposite in the Zone II of Brenner. Combination of these findings suggested a younger age for the base of Zone II than middle early Albian, where samples of this study belong to, indicating that studied interval corresponds to a part of this supposed break.;The Glen Rose Formation is also important in terms of paleoclimate. Palynofloral examination of the flora showed that Classopollis, which is pollen of the extinct conifer family Cheirolepidiaceae such as Pseudofrenelopsis and Frenelopsis that are seen in the Glen Rose Formation as megafossils, is strikingly abundant in all samples. This abundance of Classopollis pollen together with occurrence of some other pollen taxa showed a deviation from expected occurrence of these species based on the information on palynofloral provinces in Early Cretaceous, which may indicate (i) a transitional climatic zone between Southern Laurasian and Northern Gondwana provinces, (ii) special local environmental condition (saline lagoonal environment), or ( iii) reflections of a short duration of warming associated with the Aptian-Albian Oceanic Anoxic Event (OAE 1b) in the Glen Rose Formation.