The Secret of Zircon: I Know What Dinosaur Fossils Went Through

More than 70 million years ago, tens of thousands of duck-billed dinosaurs lived in the Zhucheng area (now Weifang City, Shandong Province), and then experienced a catastrophic mass death, with their remains exposed and rotting in the wild. Under the action of mudslides, these remains were gathered in low-lying areas such as Dinosaur Stream and Zangjiazhuang, and were quickly buried.

Dinosaur Fossil Gallery in Dinosaur Stream (90% of the fossils are duck-billed dinosaurs)

(Image source: provided by the author)

The remains became fossils after a long period of diagenesis. The crustal movement many years later exposed these fossils to the surface, and their mystery was gradually revealed through the excavation of scientists.

**New questions followed one after another. What was the route of the dinosaur fossils being "transported"? **Due to the long history, this question seems to be impossible to verify. However, zircon dating, a new method for determining the age of rocks, has solved the mystery for us.

Zircon: A Treasure Mineral from Igneous Rocks

Before we learn about zircon dating, let’s first take a look at what zircon is.

Zircon is a silicate mineral that is produced as a secondary mineral in various igneous rocks (rocks formed when magma cools). It is also the main ore for extracting metallic zircon. Zircon is chemically stable and is often preserved in drift sand and appears as debris in sedimentary rocks and sedimentary metamorphic rocks.

Red zircon (Image source: Wikipedia)

There are many kinds of zircon, and different zircons have different colors, including white, orange, brown, green or colorless and transparent, etc. Gem-grade zircon after cutting looks very similar to diamond.

How does the “zircon dating method” work?

Because zircon is widely distributed and has stable chemical properties, it is often used in "isotope dating" to determine the age of rocks.

The basis of isotope dating is the law of radioactive decay. By measuring the isotope content of the parent body and its daughter bodies produced by decay, the decay law can be used to calculate the time since formation, that is, the "age".

There are two types of isotopes in nature: one is radioactive isotopes (parents), which can spontaneously decay to form other isotopes (daughters). This process is called radioactive decay.

The other is stable isotopes, which do not spontaneously decay to form other isotopes or whose isotope abundance changes are negligible due to their long decay period. Radioactive decay has a certain period and is generally not changed by the environment, which is why radioactivity can be used to determine the age.

The law of radioactive decay is that the number of atoms that decay per unit time is proportional to the number of atoms of the existing radioactive parent body.

By simply measuring the parent/daughter isotope ratio accurately, the age of the material can be calculated.

The "zircon dating method" uses the principle that U and Th isotopes in zircon decay into Pb isotopes.

Zircon is relatively rich in radioactive elements such as U and Th. It contains almost no Pb initially and only forms Pb after decay. Zircon is also very resistant to erosion, so it is the best mineral for dating. It is widely used to determine the peak age of various high-level metamorphisms and the crystallization age of igneous rocks, providing chronological information for regional tectonic magmatic activities.

In recent years, researchers have used the rich age information in zircons, combined with the characteristics of the parent rocks around the basin, to identify the source of materials in different periods within the depression, that is, the source of materials for sedimentary rocks in the sedimentary area.

In a sedimentary basin, the age combination of zircons in sand bodies supplied by the same source must be the same, while the age combination of zircons in sand bodies supplied by different sources has obvious differences.

By measuring the U-Pb age spectrum of single-grain zircon in sediments within a sedimentary basin, we can effectively trace the background and properties of the source area and obtain the intrinsic connection between basin subsidence and important tectonic times.

Determining the provenance of the Zhucheng dinosaur fossil layer, zircon dating method shows its ability

The central part of Shandong is divided into two parts, eastern Shandong and western Shandong, by the Tanlu Fault Zone. The Jiaolai Basin is located in the eastern Shandong region to the east of the Tanlu Fault Zone. The southeastern part of the basin is the Sulu terrane, and the northern part is the Jiaobei terrane.

The Zhucheng Sag is located in the southern part of the Jiaolai Basin, where the Cretaceous to Cenozoic strata are mainly developed. The Cretaceous system is mainly composed of the Lower Cretaceous Laiyang Group, Qingshan Group and Upper Cretaceous Wangshi Group from bottom to top. Among them, the Hongtuya Formation of the Upper Cretaceous Wangshi Group is the main burial layer of dinosaur fossils. The middle and lower parts of the Hongtuya Formation are interlayers of sand and conglomerate of alluvial fan-braided river phase, and a large number of dinosaur bone fossils are buried in the debris flow and flood plain deposits.

Distribution of magmatic rocks in Shandong Province and overview of stratigraphic outcrops in Zhucheng area (Image source: Geological Review)

In 2016, in order to further explore the hadrosaur fossils in Zhucheng area, geological experts An Wei, Kuang Hongwei, Liu Yongqing and others from the Institute of Geology, Chinese Academy of Geological Sciences selected 6 sandstone samples containing detrital zircons from the dinosaur fossil layer at the bottom of the Red Soil Cliff Formation of the Upper Cretaceous Wang Group near Kugou-Dinosaur Stream, Zangjiazhuang and Kugou for detrital zircon dating. (Most of the zircon particles are angular to sub-angular, indicating that they have not been transported over long distances.)

It is these six sandstone samples that have helped geological experts verify the origin of the Zhucheng dinosaur fossil layer from many aspects.

The research shows that the main source of the Wang's Group dinosaur fossil layer sediments in Zhucheng area comes from the Qingshan Group volcanic rocks in Laiyang in the north and the Yishu fault zone in the northwest, and the Qingshan Group volcanic rocks on the southern edge of the Zhucheng Basin are the secondary source area.

Evidence 1: 72% of the zircons in the 536 detrital zircon grains of the six samples are of Mesozoic age, which is consistent with the age of the volcanic rocks of the Qingshan Group and the age of the granites in the north and south.

Evidence 2: The gravel components in the fossil-bearing conglomerate of the Wang Group are almost entirely volcanic rocks and pyroclastic rocks, while the Qingshan Group is also composed of volcanic rocks, which excludes the possibility that the southern and northern granites are the source areas.

Evidence 3: The ancient water flow of the Zhucheng Wangshi Group dinosaur fossil layer as a whole mainly came from the north and northwest, with a small amount coming from the south. The Qingshan Group volcanic rocks are mainly distributed in the northern Laiyang area in Shandong, with a small amount distributed in the southern Zhucheng and Yishu rift zone, and scattered distribution in western Shandong.

Moreover, the large number of dinosaur bone fossils preserved in the late Cretaceous Wangshi Group in Laiyang area to the north are similar to the dinosaur fossils in Zhucheng area in terms of fossil types and burial layers. The types are mainly duck-billed dinosaurs, in addition to horned dinosaurs, ankylosaurs, coelurosaurs, tyrannosaurus, sauropods and other types of dinosaurs.

The main strata where the Zhucheng dinosaur fossils are buried are the same as those in the Laiyang area, both of which are the Red Soil Cliff Formation and are alluvial fan deposits. The primary condition for the formation of a large alluvial fan is orogenic movement or strong block movement. Tectonic movement leads to enhanced erosion of the sediment source area (Jiaobei terrane and Laiyang area) and increased river energy. Especially when the uplift speed exceeds the riverbed incision speed, it is more conducive to the formation of a thick alluvial fan.

According to the material provenance and ancient water flow direction of the Wang's Group fossil layer in Zhucheng area, it is shown that during the deposition period of the Wang's Group in the Late Cretaceous, the paleogeomorphic background was generally high in the north and low in the south. In particular, the Laiyang in the north, the Yishu fault zone in the northwest, and the Early Cretaceous Qingshan Group volcanic rocks on the southern edge of the basin may have suffered tectonic uplift to form an erosion zone, thus providing the material source for the deposition of the dinosaur fossil layer of the Wang's Group in the Late Cretaceous.

Sediment source map of the Wangshi Group dinosaur fossil layer in Zhucheng, Shandong Province (1-sedimentary area; 2-dinosaur fossils; 3-Tanlu (Tancheng-Lujiang) fault; 4-province direction; 5-paleo-erosion area; 6-paleo-current; 7-gravel component. A-andesite; B-tuff; C-rhyolite; D-basaltic andesite; E-tuffaceous sandstone; F-tuff breccia)

(Image source: Geological Review)

Thanks to zircon dating, the story of these "poor" dinosaur remains gradually became clear: at the end of the Late Cretaceous, earthquakes occurred frequently and volcanic activity was intense, leading to frequent mudslides. As a result, a large number of alluvial fans and braided river deposits were formed along the Jiaobei terrane and Laiyang area in the north toward the Jiaolai Basin. In such a harsh environment, the remains of the dinosaurs that died in groups were washed and transported by floods or mudslides from north to south, and were eventually buried collectively in the Zhucheng area.

The world has changed dramatically. Now, with the advancement of technology and the efforts of scientists, we are able to explore the details of what happened tens of millions of years ago.

References:

1. An Wei, Kuang Hongwei, Liu Yongqing, et al. Detrital zircon dating and provenance tracing of the Late Cretaceous Wang Group dinosaur fossil beds in Zhucheng, Shandong Province[J]. Geological Review, 2016, 62(2): 453-471.

Produced by: Science Popularization China

Produced by: Sun Jiafeng (Zhucheng Dinosaur Culture Research Center)

Producer: Computer Network Information Center, Chinese Academy of Sciences