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Dinosaurs in the Land of Genghis Khan: The Geologic Evolution of Southeastern Mongolia

Cari Johnson
Geological and Environmental Sciences
Stanford University
May, 2001

About 50 million years ago, the Indian subcontinent slammed into the southern margin of Asia, resulting in the uplift of the highest mountain chain in the world, the Himalaya. Amazingly, this is only the most recent of a series of similar collisions that have occurred in this region over the last 300 million years. The result of these plate tectonic events is the Asian continent, a virtual jigsaw puzzle of small blocks that are glued together along the remnants of Himalayan-style collision zones. For a geologist, Asia is a unique natural laboratory in which to study the birth and growth of a continent.

Rather than studying the entire history of this enormous region, I concentrate on a relatively small corner of southern Mongolia, where I study rocks that formed during the late Mesozoic (150-90 million years ago, the time of the dinosaurs). My colleagues and I use various observational and analytical techniques to interpret the processes that formed the rocks (e.g., rivers, lakes and volcanoes), and demonstrate how these processes changed through space and time. In addition to improving our understanding of a poorly known region, this research addresses larger-scale questions regarding the petroleum potential of southern Mongolia and the tectonic evolution of the Asian continent.

Mongolia is presently in the geographic center of Asia, but it used to be along the southern margin of the continent, facing an ocean. Around 250 million years ago, a small tectonic plate, called the North China block (which includes the land on which Beijing now sits), collided with Mongolia, closed the ocean between them, and formed a mountain range. This process is similar to squeezing together the bellows on an accordion: ridges form and push up into mountain belts, and the crust becomes compressed and thickened. Soon after, this same crust begins to pull apart, creating a series of mountain ridges separated by areas of low topography, just like the folds in an accordion's bellows when it is extended. For about 60 million years, sediment eroded from these mountain chains and collected in the low-topography areas that we call sedimentary basins. This same area was once again pushed together around 90 million years ago, probably as a result of yet another collision. Fortunately for us, the sedimentary basins that record the extensional period were tilted up and exposed in some places, where we can study them now.

Southern Mongolia was clearly a very dynamic place during the late Mesozoic. Since sedimentary rocks and preserved basins are our only records of what happened during this time, my work focuses on understanding their evolution. This exercise has many diverse implications, including the potential for natural resources. Oil was discovered in southern Mongolia during the 1950s, and further exploration and production of this resource could be of considerable value to the Mongolian economy. All of our evidence so far indicates that this petroleum system is driven by the late Mesozoic extensional sequence that I study. For example, we have used the oil to look for molecular-scale fossil remnants of organisms that produced and processed organic matter, such as algae and bacteria. By analyzing the geochemistry of the oil, we have determined that the source of the petroleum is organic matter that was buried in lake deposits. Thus, understanding the locations and characteristics of these ancient lakes helps us to predict the distribution of lacustrine source rocks, an important aspect of the petroleum system.

In addition to economic interests, this work also addresses certain questions related to the tectonic evolution of Asia. For example, why did this region start to pull apart so soon after a major collision? To answer this, we need to constrain the timing of extension, understand how the landscape evolved, and then search the geologic literature for possible explanations. We now know that the northern margin of the Himalayan mountain belt (a collision or contraction-related structure) is actually an extensional fault. It appears that mountain belts can grow to be so big that they are gravitationally unstable; the thickened crust cannot support its own weight, so the mountain belts collapse and extend as a result. This is only one of many possible causes for extension in my field area.

I attempt to answer these research questions by combining fieldwork, taking samples, and laboratory analyses. Fieldwork is similar to detective work in that we study the remnants of ancient landscapes and look for clues to help us create snapshots of the basins at different points in time. I travel to specific outcrops in Mongolia where rocks of possible late Mesozoic age are exposed, and attempt to identify the processes that formed them. Geologists often say, "The present is the key to the past." In other words, we can use observations of modern sedimentary environments, such as lakes and rivers, to identify what environments are preserved in the rock record. In the case of ancient rivers, we can often tell in which direction they were flowing by looking for ripples and other current-flow indicators in the rocks. This gives us an idea of the locations of mountain ranges that fed these rivers, or the “paleo-geography” of the area.

Another important aspect of fieldwork is taking samples; we bring back literally tons of rocks that are analyzed by various procedures in several different laboratories. One of the most important methods I use is to determine the age of a rock by measuring radioactive decay of certain isotopes. This procedure helps us determine relative ages of different rock exposures, which we combine with our field observations to determine how the system changed through time.

Finally, my ultimate goal is to integrate this information, compare notes with other geologists working in the area, and try to draw some conclusions. This part of my project requires some creative thinking to infer what this region looked like millions of years ago, and to suggest why it formed. Although Mongolia may seem like an exotic and somewhat obscure study area, the history of this region has many connections with geology worldwide. For example, a large fault cutting through my field area appears to have mainly strike-slip motion, which is the result of plates sliding past each other such as along the San Andreas fault system in California. Also, areas in the western United States (particularly Nevada and Utah) have been pulled apart much like Mongolia, resulting in a series of sedimentary basins separated by mountain ranges. Thus, much of the western US is believed to be a modern analog for the extended region of Mongolia. Therefore, the results of this work will not only present a window into the past history of Asia, but may also reveal how modern systems will evolve through geologic time.