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The geological setting here presented is summarized from Bureau of Geology and Mineral Resources of Guangdong Province(1988)and Zhang et al.(in review).The Heyuan Basin is a fault basin,which was controlled by the main geological structures of the Heyuan Major Fault(Fig.2).The Heyuan Major Fault was caused by the Indosinian Movement,which took place at the close of mid-Triassic,forming a series of largely NNE-and NE-trending folds in mid-Triassic and older strata in company with faulting,magmatism and metamorphism.It was also deformed in the Yanshanian Movement.The 1st phase of Yanshanian Movement occurred at the end of the Early Jurassic.In the mid-Jurassic,lacustrine clastics and volcanics filled fault basins in eastern Guangdong,forming the Zhangping Formation,whereas in the northern part of this province,the equivalent stratum is the Maziping Group of the intermontane basin facies.The 2nd phase of Yanshanian Movement occurred at the end of mid-Jurassic,giving rise to a broad open folding involving the Mid-Jurassic and older strata accompanied by magmatic activity and metamorphism on considerable scale.With further strengthening of plate subduction in the Late Jurassic,the 3rd phase of Yanshanian Movement took place.Thus,many deep and large faults in this province be came active again in company with the neogenic faults,forming widespread NE-trending volcanic zones.This eruptive activity is especially intense and widespread in the coastal area of eastern Guangdong.The 4th and 5th phases of Yanshanian Movement,occurring at the ends of Early and Late Cretaceous respectively,were both accompanied by volcanism and plutonism,but the inten sity was much lower than that of the Late Jurassic igneous activity,suggesting that the Yanshanian Movement was then in its waning stage.In the Early Jurassic,with the subduction of the Kula-Pacific Plate under the Eurasian Plate,the sea invaded central and northern Guangdong,leading to the extensive sedimentation of marine deposits of the Jinji Formation(Bureau of Geology and Mineral Resources of Guangdong Province,1988).At the end of Late Jurassic,large-scaled plutonism took place,resulting in the high-temperature,and low-pressure metamorphic zone and a migmatite zone along Changle-Nan"ao deep fault zone.Beginning with the Early Cretaceous,this province entered a tectonic stage of relative relaxation,brought about by the appearance of a series of medium-to small-sized fault basins controlled by tensile faults.At the same time,a suite of red clastics with volcanics was laid down in the basins.This sedimentation continued into the Late Cretaceous when a gypsum-and salt-bearing red-colored sequence was formed(Bureau of Geology and Mineral Resources of Guangdong Province,1988).
Fig.2 Map of the position of the stratigraphic section in northern Heyuan Basin
Heyuan Basin is one of the ten basins in Guangdong Province,and occupies about 725 km2,consisting of the Upper Cretaceous and Lower Tertiary deposits(Zhang et al.,in review).It extends from the southwest to the northeast.It is an irregular“S”shape.The northern part of the Heyuan Basin is crescent-like,in a north-northeast direction,and is located in north of Datangpu village,which is a suburb on the northern side of the Heyuan Basin.The northern part of the basin is 21 km long from north to south,6~8 km wide from east to west,about 160 km2.The Heyuan Major Fault is located at the northwest side,and a northeast oriented reverse fault is found on the east side.The strike of the beds is north-north-east,and the angle of the inclination is 24°~38°.Due to the long-term activity of the Heyuan Major Fault on the northwest side,the sedimentation center is inclined to the west side of the basin and it shifts continuously from the southeast to the northwest,forming a monoclinal basin,which dips to the northwest.The thickness of the Cretaceous-Paleocene strata of the northern part of the Heyuan Basin reaches more than 4000 m.These sediments are in unconformable and fault contact with the Jinji Formation of the Early Jurassic and the third stage of Yanshanian granite(Fig.3).
The red beds of the Cretaceous and Paleocene in the northern part of the Heyuan Basin are a series of coarse red colored clastic rocks,characterized by conglomerates,sandy gravels,and mature heterogranular sandstones with granule gravel.In ascending order,they are divided into the Dafeng Formation,Zhutian Formation,and Zhenshui Formation of the Nanxiong Group,the Shanghu Formation and the Danxia Group of Paleocene-Eocene(Fig.4)(Zhang et al.,in review).
The Dafeng Formation(K2d)is distributed in Datangpu,Tianxin,Mazhuwo,Chentian,Luowu and Daoji,near the southeastern margin of the northern part of the basin.These areas are located in the southern and eastern suburbs of Heyuan City.The formation is characterized by coarse sediments,mature conglomerates and granule gravels,from coarse at the bottom to fine at the top,comprising two depositional cycles,837 m in thickness.Near the base of the Dafeng Formation consists of dark purplish red massive boulder conglomerates,and is poorly sorted.Eighty percent of the conglomerates are slightly metamorphosed,with pebbly sandstone lenses.It contacts the third stage of the Yanshanian granite by fault or lies unconformably on the Jinji Formation of Late Jurassic age.Its thickness reaches 237 m.The lower part of the Dafeng Formation is 200m thick,and is composed of purplish pebble gravels bearing unsorted sandstone and medium-fine sandstone.The rock is strongly weathered,and the topography is low and flat and often covered with vegetation.The middle part of this formation is formed of purple-red conglomerates interbedded with sandy conglomerates.The Danxia landform is distinctive.This part is 150 m thick.The upper part of the formation consists of purple-red microconglomerate with medium-coarse granitic debris in fine gravels and gravelly unsorted sandstone.The conglomerates are granitic in composition,with lesser amounts of sandy shale from the underlying Jinji Formation.Round dinosaur eggs are abundant in the upper Danxia Formation.The formation is 250 m thick.
Fig.3 The stratigraphic section from the Shengli fluorite mineral deposits to Niuguzhai in the northern Heyuan Basin
The Zhutian Formation(K2z)crops out in Heyuan Urban District,Huangsha,Dongyuan County,and Xuantang town.It is characterized by relatively fine sediment sizes(Plate I),differing from the underlying Dafeng Formation and the overlying Zhenshui Formation.This formation is composed of purple-red and light red microconglomerates,medium-coarse grained feldspathic qu artzose sandstone and heterogranular sandstone forming interbeds of varying thickness.The lower part is banded with silty-fine sandstone and ferruginous mudstone.Widely distributed gray-green mottles and calcareous concretions are present in the sandstone,which has well developed cross bedding.The sandstone is easily weathered.The topography is low with gentle hill slope;turtles,Heyuannia huangi(Lü,2003),and elongated dinosaur eggs were found in this formation.The formation is 1200 m thick.
Fig.4 Stratigraphy in the Heyuan Basin(Abbreviations see Fig.5)
The Zhenshui Formation(K2zs)is distributed in the area of Liantangping,Weibei,Bugouyan,Maguding,Yangmeiping and Hezitian and south to the 205 National Road from Baitouling to Shenpu.Sediments are relatively coarse,with white,purple-red thick-bedded and blocky microconglomerate containing medium pebbles,gravel-bearing sandstone interbedded brown-purple feldspathic quartzose sandstone and fine sandstone.The sandstone contains small gray-green mottles.Well developed cross-bedding is present in the upper part,but no fossils are found here.The formation is 900 m thick.
The Shanghu Formation(E1s)is distributed in the areas of Meishixia,Huangguotang,Pingshan,Changlong,Maopu and Sanfutian.It consists of purple-red and reddish orange microconglomerates with abundant granite conglomerate,gravel-bearing coarse sandstone,and feldspathic quartzose sandstone,partly intercalated with pelitic siltstone.The most common composition of the gravel is granitic,followed by quartzose sandstone and the underlying red sandstone of the Nanxiong Group.The gravel is well rounded.The sandstone has well-developed cross-bedding,and the thickness of the cross-bedded section of beds is less than 1 m.No fossils were found in this formation,and it is 820 m thick.
The Danxia Group(E2dx)is distributed in the northwest margin of the basin,which includes the areas of Meishizhai,Goubili,Niuguzhai and Shimen.It consists of purple-red,dark-red,brown-red massive conglomerates and medium-coarse granitic sandstone bearing microconglomerates.The upper part is dominated by microconglomerate while the lower is megaconglomerates,with grains exceeding 1 m in diameter.Most gravels are granitic,poorly sorted and poorly rounded.They are mixed with ferruginous,pelitic and sandy materials.Silicification is clear near the faulted areas.The rock is hard,often forms an overhanging cliff,and is termed the Danxia Landform in Meishizhai,Niuguzhai and Shimen.No fossils were found in this formation.This group is 534 m thick.
The Nanxiong Group of the northern part of the Heyuan Basin is divided into three units based on changes in sediment size(from coarse to fine,and fine to coarse again).The lower part,the Dafeng Formation is characterized by coarse lithology,mature conglomerates,clear topographical expression,and rapid sedimentation.This represents the early stage of basin development.The sediments of the middle part,the Zhutian Formation,are relatively fine and rich in gray-green calcareous concretions or calcareous patches.This unit was deposited in environments representing relatively a wide,deep basin,with distant source areas,and low basin margins.The lithology of the upper unit,the Zhenshui Formation,becomes coarse again.It is characterized by mature conglomerates.The above-mentioned characters are similar to those of Nanxiong Group in its type area of the Nanxiong Basin.These two units can be correlated(Fig.5).The main difference between them is that the clast sizes of the Zhutian Formation and the Zhenshui Formation in the Heyuan Basin are relatively coarser than those in the Nanxiong Basin.Microconglomerates,gravel-bearing sandstone and unsorted sandstone are relatively common,but silty mudstone and pelitic siltstone are less common.Fine clastic rocks and clays are well represented in the Nanxiong Basin.Notably,the Zhutian Formation consists of a set of fine siltstones and silty mudstones and interbedded lenticular limy sandstone and marl.Clays are also relatively common.The Dafeng Formation of the Nanxiong Group in both basins is rich in dinosaur eggs.Oviraptorids(Lü,2003)are found in the Zhutian Formation of Heyan Basin.Numerous kinds of fossils were found in Nanxiong Group of the Nanxiong Basin(Young,1965;Zhao,1975;Zhao et al.,1991;2002).Oviraptorids are also found in the Zhutian Formation of Wujing and the top of the Zhenshui Formation(top of the Pingling Formation,Zhao Z-K.personnel communication to Lü,2003)near the New City District of Nanxiong.In addition to reptiles,microfossils,such as Ostracoda,Foraminifera,charophytes(Zhang,1987),and gastropods,lamellibranchs,and insects(Zhang et al.,2000a,b)are abundant,whereas the biota of the northern part of the Heyuan Basin is limited,producing no microfossils,mollusca or arthopoda.The biota is much less rich and diverse than that of the Nanxiong Basin.
Fig.5 Stratigraphic correlation between northern part of the Heyuan Basin(A)and Nanxiong Basin(B)
The Shanghu Formation of the Nanxiong Basin is dominated by purple-red,dark purple-red silty mudstone and pelitic siltstone,rich in calcareous concretions,and interbedded fine sandstone,partly intercalated with pebbly sandstone and microconglomerates,abounding in fossils(Zhang,1992a).In the northern part of the Heyuan Basin,the Shanghu Formation is dominated by microconglomerates,pebble-bearing coarse sandstone,heterogranular feldspathic quartzose sandstone,and rare silty mudstone and fine sandstone.No fossils were found in this formation.The sediments are clearly coarser than correlative ones in the Nanxiong Basin.The Danxia Group is a set of coarse clastic rocks.It is coarse in the lower part,but fine in the upper part.This group is typified by the Danxia Landform,and it is similar to the Danxia section of the type locality in the Danxia Basin.The lithological and topographical characters can be correlated.The Danxia Group of the Danxia Basin is Late Cretaceous(Zhang,1992b),but the age of Danxia Group in the Heyuan Basin is not constrained,and it may be younger than in the Danxia Basin.The Danxia Group near both margins of the Nanxiong Basin is the facies equivalent of the Nanxiong Group and the Luofuzhai Group.It is an obviously diachronous stratigraphic unit.Its age ranges from the middle Late Cretaceous to the Early Eocene.From the west to the northeast toward Shitouzhai,the units of the Danxia Group become progressively younger(Zhang et al.,2000a,b).In short,no fossils are found thus far in the Shanghu Formation and the Danxia Group in the Heyuan Basin,but they lie above the Nanxiong Group,their stratigraphic sequence is clear,and their age should be younger than that of the Nanxiong Group.