南阳盆地西峡赵营村上白垩统高沟组恐龙蛋化石埋藏特征及古环境意义

The Dinosaur Egg Embedding Characteristics and Palaeoenvironmental Significance for the Upper Cretaceous Gaogou Formation in Zhaoying Village,Xixia County,Nanyang Basin

豫南南阳盆地西峡赵营村上白垩统高沟组含有较多的恐龙蛋化石,其中最为著名的是“路易贝贝”胚胎化石。对含化石地层中恐龙蛋的保存特征和古气候重建,可进一步认识白垩纪恐龙灭绝的古环境背景和原因。本文实测赵营村剖面厚约267.8 m,主要由下伏泥盆系南湾组(厚7.9 m)、上白垩统高沟组(厚259.9 m,未到顶)组成。通过地层沉积物结构构造观测和样品采集,在室内进行了元素、碳酸盐碳氧同位素组成、有机质饱和烃生标化合物等测试。结果表明,高沟组沉积物主要源于熊耳山岩浆岩的风化和短距离迁移,恐龙蛋化石主要保存于海拔200~800 m的山麓至平原过渡带的河漫滩环境,以原地埋藏为主。降雨量的变化造成河漫滩与河道频繁变化,从而导致恐龙在山麓与河漫滩之间来回迁栖。生物标志化合物分析表明,含恐龙蛋化石地层有机质主要来源于水生植物、菌藻类和陆生植物,但植被总体不发育。利用地层沉积岩Si/Al、CIA、Rb/Sr比值,以及δ13C、δ18O等环境指标对高沟组古气候重建,该时期发生了较为快速的旋回变化,经历了暖干—暖湿—干旱—暖湿—干旱的古气候演变过程,总体上呈逐渐干旱化的趋势。通过赵营村晚白垩世早期恐龙蛋化石的埋藏、以及地层的古环境重建,说明古气候、古环境状况可能是影响恐龙生存的主要因素,大量恐龙蛋不能完全孵化以及气候的干旱化导致植被恶化可能是恐龙逐渐消亡的原因。

Lots of dinosaur egg fossils,especially famous“Baby Louie”dinosaur egg embryo fossils,exist in the Upper Cretaceous Gaogou Formation in Zhaoying Village of Nanyang Basin,Henan Province.Based on sampling and proxy analysis from the section of embedding egg fossils,the authors constructed palaeoclimate and palaeoenvironment and investigated paleoenvironmental background and reason for dinosaur extinction in the Cretaceous.A 267.8 m section was determined in the Zhaoying Village,which is made up of Upper Cretaceous Gaogou Formation overlying Devonian Nanwan Formation.Based on analysis of bulk sediment elements as well as carbon and oxygen isotopes,the authors detected that sediments in the Gaogou Formation mainly came from the magmatic rocks from the Xionger Mountain.Dinosaur egg fossils embedding in situ were kept in the flood plain belt near the foot of the mountain to the far plain with a range of 200 to 800 m.Runoff in the river and surface came from precipitation.River and flood plain environments changed quickly with the changing precipitation so that dinosaurs migrated between the foot of the mountain and the flood plain.Under the warm and wet condition,higher runoff formed the flood plain sediment environment with muddy silt rocks,which was favorable for the dinosaur living,egging and hatching.When the precipitation rose,flood plain environment was covered by the river and made dinosaur migrate back.Judging from the survived complete dinosaur egg fossil and the proxies significance,the authors hold that river hydrodynamic action was not very strong and ascended slowly.Organic matter in the strata containing dinosaur egg fossils came from germ,alga,and aquatic and terrestrial plants,but with poor vegetation as shown by the biomarker analysis.Geochemical proxies Si/Al,CIA,Rb/Sr and carbon and oxygen isotopes of carbonates show that paleoclimate of the Gaogou Formation changed quickly as a warm/dry–warm/wet–dry–warm/wet–dry process,and had a drying tendency.Under the dry condition,precipitation and effective humidity decreased,runoff decreased also with rare vegetation cover so that huge animals could not find enough food for living.Based on the research on the dinosaur egg embedding and Late Cretaceous paleoclimatical reconstruction for Zhaoying Village of Xixia County,Henan Province,it is held that the main factors causing gradual extinction of dinosaur were the palaeoclimatical and paleoenvironmental deterioration which made lots of dinosaur eggs hatch incompletely and vegetable decreased largely.