Every year, thousands of people visit the Petrified Forest National Park in northern Arizona and are astonished by the beauty of the fossil wood there. In the Painted Desert, almost every piece of petrified wood appears unique. Some show perfect preservation of the original wood structure, as if the trees were still living there yesterday. Some are like giant crystals, displaying a “rainbow” of colors with red, orange, yellow and milky white etc., sparkling in the sunlight as if covered by glitter. Apart from its aesthetics, however, little did we know is the rich history of geology being “locked” in these fossils.
The story of Arizona’s petrified wood started about 225 million years ago during the Late Triassic period, when dinosaurs and large reptiles dominated on Earth. The land within present-day Arizona was at the time a lush subtropical forest filled with the ancestors of present day conifers. Episodically, rivers were flooded by rain storms, which washed mud and other sediments into the lowlands. These sediments are now preserved as a package of colorful sedimentary rocks known as the Chinle Formation. During Late Triassic, at the time when the rivers were active, fallen trees and broken branches were often buried by river sediments quickly enough that oxygen was cut off and decay was slowed down.
Eruptions from nearby volcanoes blanketed the area in volcanic ash, which introduced silica into the groundwater. In the buried trees, their cellulose and lignin (components of wood) had an affinity of silica. As groundwater flowed through, the wood absorbed silica through a process known as “capillary attraction” allowing fossilization to occur. Silicification of the wood involves silica replacement of the original woody tissue as well as the filling of pore spaces in the intracellular structure, known as permineralization. The silica in the wood started as silica gel (opal), as time went by, it transferred into a more crystalline form such as quartz as the water in the gel gradually dried out. The quartz within the petrified wood is hard and brittle, fracturing easily when subjected to stress. This crystal nature of quartz created clean fractures, evenly spaced along the tree trunk, giving the appearance today of logs cut with a chainsaw.
Si precipitation in the wood started rapidly after burial. But there wasn’t just one single episode of quartz precipitation. As wood is porous, much like bones, open pathways within the wood may persist over long periods of time, allowing some parts of the wood to remain permeable for groundwater flow. Within a given sample, difference in the textures, colors and distribution of trace elements such as Fe suggest that solidifying wood was exposed to penetrating solutions with different environmental conditions over the course of time.
The fact that petrified wood can be formed through multiple episodes of quartz precipitation makes it a potential recorder of the geologic history of the place where it was formed, and in particular, history of groundwater activities. When were the major episodes of groundwater event that brought in Si-rich water that precipitated quartz in the wood? Under what environmental and/or tectonic conditions can we have such groundwater events? To extract this information from the fossil wood, we need some help from geochemistry.
Along with silica, dissolution of volcanic ashes also releases other trace elements, such as uranium (U). U is an interesting element in the way that it is redox sensitive, which means that it has different mobility (how easy it can be dissolved in solution and being transported by fluid) in environments with different oxygen level. Here, the word “oxidizing” means more oxygen, and “reducing” means less oxygen in the environment. In the oxidizing conditions of aqueous systems, U is converted into its oxidized and soluble species, allowing it to be mobilized from volcanic ashes. However, when these oxidized, U-bearing and Si-rich waters contact reducing materials, such as ferrous Fe, sulfide or organic carbon (such as remains of the wood), the U is reduced into a more insoluble state and immobilized. Recall that the wood organic matter would absorbed silica? In this process U is precipitated along with silica, and we get U-rich quartz crystallized at different times in the petrified wood.
What is cool about U is that it is also an radioactive element, which means that it is unstable and will give off radiation in the effort to stabilize itself. In the U-rich quartz in the petrified wood, nucleus of U disintegrated into lead (Pb) and helium (He) in proportion. U decay has a fixed rate, therefore quartz formed at different times will give different U/Pb ratios. At Rice University, we used Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) to analyze U and Pb isotopes in the petrified wood to determine the age of quartz, which will in turn tell us when the oxidized groundwater events took place. The technique has the advantage of focuing a laser beam with small size (i.e. 80 μm) on different parts of a given rock sample, so we can obtain in-situ results from quartz with different crystallization histroy in the same sample.
Our analyses show that petrified woods from the Petrified Forest National Park record a 200 million-year long interval of subsurface hydrology in the park area in the southern Colorado Plateau. There are major peaks of the dates, clustered within ~220, 180, 140 and 40 Ma. The Chinle Formation, which is the sedimentary unit containing these petrified wood, was deposited between 225-205 million years ago. The ~220 Ma age cluster thus suggests initial fossilization of the wood took place shortly after the wood was buried in the river sediments. As the fossil wood may not become completely impermeable after initial silicification and can still serve as fluid pathways, the younger U-Pb dates record subsequent groundwater events during which the petrified wood underwent further permineralization.
What do these groundwater events tell us about the geologic history of the southern Colorado Plateau? Or, how can we get oxidized groundwater flow through the strata to mobilize U from volcanic ashes and precipitate it in the petrified wood? The simple hypothesis is that because there is more oxygen towards the surface, we can change the redox condition of the strata by moving them up and/or down. When a region is undergoing subsidence and therefore submerged within an inland sea or lake, strata like the Chinle Formation might remain waterlogged and hence lie below the groundwater redox transition zone and be reducing. On the other hand, regional exhumation would force rocks towards the surface, exposing rocks to oxidizing groundwaters or even the oxidized vadose zone. In fact, except for the major ~220 Ma peak, younger age peaks at 180-170, 150-120 Ma, and 80-20 Ma appear to coincide with development of major regional unconformities, which represent breaks in the sedimentary record, as missing pages in our Earth history book. These unconformities are often associated with exhumation events.
Let’s zoom out a bit and take a look at what happened in the northern Arizona area since Late Triassic. To the west, there was subduction of the oceanic Farallon Plate underneath the continental North American Plate. Convergence of the two plates resulted in intense compression and thickening of the crust, leading to development of a series of mountain belts along the present-day western Utah and eastern Nevada region. The immense mass from these mountains caused the lithosphere to bend, or flex, downwards. As a result, a structural basin called foreland basin was developed adjacent and parallel to the mountain belt, receiving sediments that were eroded off the mountains. Just imagine you put some weight on one end of a thin slab while holding the other end of this slab. The slab will be bent towards the end with the weight, and a bulge will be developed on the other end to balance the force. The location and height of this bulge can be changed if you change the weight. During Late Triassic to Early Cretaceous (230-100 million years ago), the northern Arizona area was located on a migrating forebulge, as a response of changes in the development of mountain belts in the west. Activities of forebulge during 180-170 Ma and 150 Ma led to exhumation and prolonged subaerial exposure in this area, which introduced oxidized groundwater into the Chinle strata.
In the late Cretaceous (~100 Ma), continuous loading of mountains and subsidence of the foreland basin led to the development of a broad, shallow inland sea in the western North America, known as the Late Cretaceous Western Interior seaway. With time, subsidence caused the Western Interior Seaway to expand into the Four corners region. In northern Arizona, the Chinle Formation was completely submerged and buried, during the maximum sea level rise between 100-90 Ma. Being buried in a reduced environment, there was no/little U mobilization in the Chinle Formation, resulting in a sparsity of U-Pb ages of this time period in the petrified wood.
From the Late Cretaceous to mid-Cenozoic (80-20 Ma), the western coast was affected by the Laramide orogeny, which partition the Western Interior into a series of small basins and uplifts. Colorado Plateau started rising at a similar time period, bring the Chinle Formation up to more oxidized environment. Oxidized groundwater started passing through the strata again and contributed to the 80-20 Ma age cluster in the petrified wood.
In the Painted Desert, fallen wood “lived” through the past 225 million years continues to tell us stories of geology and hydrology in northern Arizona. The Petrified Forest is in essence a nature-made museum of art and history.