More Information About Time (Duration) and Fossil Tree Stumps
Modern trees can be buried naturally in a wide variety of ways. In Kentucky, fossil tree stumps encased in sandstone appear to have been mostly buried in relatively rapid flooding events associated with ancient river channels or in environments influenced by tidal fluctuation. Modern river floods can deposit several feet of sediment in hours to days. This is catastrophic or episodic deposition. Fossil tree stumps encased in shale and siltstone were buried by sedimentation of mud and silts out of the water column in wetlands, lakes, bays, streams, and estuaries. Deposition of several feet of mud can also be instantaneous (for example, slumps and slides), but in standing bodies of water, generally takes longer (years to decades). The accumulation of several feet of sediment in days to decades is extremely fast in geologic terms.
Just because the fossil stump represents rapid sedimentation and burial, that does not mean that the rocks immediately above or below the stump were deposited in the same amount of time. Think about sedimentation on a floodplain. If the river floods its banks, some thickness of sediment is deposited in seconds or days on the floodplain. Then, years or decades may go by without any further sedimentation. In fact, some of the sediment that was deposited may be eroded. Thirty years later another flood happens and a sediment layer is deposited above the old flood deposit. The new flood layer takes minutes or days to be deposited. The two flood layers individually represent deposition in a short time span, but the two layers together represent a much longer time span. Net or average sedimentation rate is the mean rate across long periods of time. Sedimentation rates are commonly averaged as sediment thickness in millimeters per 1,000 years. Most rock intervals represent long periods of time.
Coal beds tend to be separated by 20 to 50 feet of sedimentary rock in sedimentary rock packages, which are sometimes referred to as cyclothems or parasequences. If we examine average long-term sedimentation rates rather than local, catastrophic events, average sedimentation rates for rivers, floodplains, and lakes are much less than an inch per year. To deposit 50 feet of sediment on average in most wetland areas with average subsidence rates would require tens of thousands to millions of years.
In the coal-bearing rocks of Kentucky (and other coal basins), coals were deposited in repetitive depositional cycles. Cyclic alternation of sedimentation occured through many mechanisms, but in Pennsylvanian, coal-bearing strata, it is generally thought to have been controlled by sea-level changes. Sea-level change occurs at many scales and across many time spans, but during recent times and during the Pennsylvanian Period, sea-level change was influenced by natural cycles in glaciation. Cycles of continental-scale glacial advance and retreat are related to natural cycles in the earth’s orbit. These cycles are termed Milankovitch cycles. Milankovitch cycles occur at scales of 20,000 to 400,000 years. The cycles are related to different astronomical aspects of the earth’s tilt and orbit relative to the sun. Interactions of the 40,000-year (obliquity), 100,000- to 125,000-year (short eccentricity), and 400,000-year (long eccentricity) cycles appear to have influenced ice advance and retreat during the last ice ages, and many scientists think the same cycles influenced sea level during the Pennsylvanian Period. For simplicity, let’s hypothesize that a 50-foot-thick, coal bed–to–coal bed cycle represents the 100,000-year orbital cycle.
First, during Milankovitch cycles, sea level rises and falls, like a sine-wave curve. The curve is asymmetric, and may have superimposed curves of smaller duration, but to simplify the discussion, we’ll assume a symmetric rise and fall. During half of the cycle, sea-level is falling and glacial ice is building up at high latitudes. During the other half of the cycle, sea level is rising as the large continental ice sheets melt. This means that in many coal-bearing rock cycles, the rocks we see preserved only represent half of the Milankovitch cycle, and erosion or nondeposition was happening during the other half of the cycle. In some cases, there would be more time for sedimentation; in others, less. For our hypothesized 50-foot-thick coal-to-coal interval, in a 100,000-year sea-level cycle, perhaps 50,000 years is preserved in sedimentation, and 50,000 years could be absent from erosion or nondeposition (with variability). Fifty thousand years of erosion may be represented by an erosional contact, or series of erosional contacts in our 50-ft-thick rock sequence. For example, the contact between a scour-based sandstone and underlying shales is an erosional contact. That contact represents some amount of eroded sediment and therefore missing time. The amount of missing time along an erosional contact may be very short or very long.
Most coal-bearing depositional cycles contain an underclay, which represents an ancient hydric soil (termed a “gleysol”) that developed with the overlying wetland. These beds are usually thin (inches to feet) but can represent long amounts of time. Dates of modern soil sequences around the world show that soil formation generally takes decades to millennia to form (see for example, Van Breemen and Buurman, 2002). An average annual rate of soil accumulation on Earth is 0.0056 centimeters/year (0.0022 inches/year) (Wakatsuki and Rasyidin, 1992), but is highly variable depending on climate, plant types, substrate mineralogy, and other factors.
The coal bed in our coal-bearing cycle represents the accumulation of peat in an ancient swamp. Most coals are relatively thin (inches to several feet) but represent a long period of time. Dates from the most extensive peatlands in the world today (western Siberia and around Hudson Bay, Canada), show these peats began to accumulate as glacial ice melted 16,000 years ago, but did not become widespread until 10,000 years ago (Kremenetski and others, 2003; MacDonald and others, 2006). Some modern peats from Southeast Asia, which are often used as analogs for Kentucky’s coal beds, began accumulating 26,0000 years ago, and became more widespread and thicker 8,000 to 13,000 years ago (Page and others, 2004). Hence, based on modern examples, the thick peats that form coal beds generally represent thousands of years of accumulation.
The time it took to bury a 5- or 6-foot-tall tree stump in the 50-foot-thick sediment cycle may have been less than a year to decades, but the longer-term sedimentation rate for the entire interval would have been longer. The net sedimentation rate is the average rate of sediment deposited over periods of time generally measured in hundreds or thousands of years. This doesn’t mean constant sedimentation at that rate; sedimentation in most depositional environments occurs in pulses separated by much longer periods of nondeposition or erosion. Net sedimentation is the net balance between deposition and erosion over hundreds to thousands of years. Modern net sedimentation rates vary from 3 milimeters to meters per thousand years, depending on many factors (Reading, 1978; Blatt and other, 1980). This equates to 0.12 inch/1,000 years (0.0001 inch/year) to more than 39 inches/1,000 years (0.04 inch/year) which is still not what most people would consider very rapid sedimentation.
If sea-level changes were the primary influence on the available accommodation space and the rate of tectonic subsidence (discussed in the next section) was equal to or greater than the sedimentation rate, then sedimentation rates for our example 50-foot-thick sediment package between coal beds can be examined relative to the space that could be created by sea-level change. If the sediment was deposited under the influence of the 400,000-year Milankovitch cycle, the net sedimentation rate (for long periods of time termed the tectonic subsidence rate) is 0.0015 inch/year. If deposited under the influence of the 100,000 year cycle, the net sedimentation rate would be 0.006 inch/year. The average sedimentation rate for the same interval if deposited under the influence of the 40,000 year cycle would be 0.015 inches/year. Of course, this assumes no major unconformities between sediment or rock layers, in the example rock interval. If major unconformities occur within the interval, that rock layer could represent longer periods of time.
Other Influences
For standing tree stumps to stay buried and not be eroded, larger-scale processes are often needed. In nature, material can stay buried and not be subject to erosion if base level rises (for example, sea-level rise) or ground level drops or subsides. Two types of subsidence can influence burial of fossil tree stumps: compactional and tectonic.
Compactional subsidence happens in areas where new sediment is deposited above pre-existing compactable sediment. In modern coastal Louisiana, for example, compactional subsidence rates vary from hundredths of an inch or less to 17 inches per year (Swanson and Thurlow, 1973; Boesch and others, 1983; Penland and Ramsey, 1990). At the high rate, only 3.5 years would be needed to bury a 5-foot-tall tree stump. At the low rate, the time would be much longer. In areas where the in-place tree is above a coal bed, compaction of the original peat that formed the underlying coal bed might have created the accommodation space to allow a fossil tree trunk to be preserved in a relatively short time. Relatively rapid compaction of many feet of peat is possible in decades to hundreds of years (see for example, Drexler and others, 2009). Peat compaction would not occur forever, however, so strata above the preserved stump in our 50-foot-interval may have taken longer to be deposited.
Tectonic subsidence happens in regions called basins. The Eastern Kentucky Coal Field is in the central Appalachian Basin. The Western Kentucky Coal Field is in the Eastern Interior (also called Illinois) Basin. Neither of these areas is subsiding today, but both were undergoing active tectonic subsidence during the Pennsylvanian Period, when most of Kentucky’s coal beds accumulated. Normal tectonic subsidence rates are also very slow, rarely exceeding 39 inches/1,000 years (0.04 inch/year). Typical rates are 0.4 to 1.6 inches /1,000 years (0.0004 to 0.0016 inch/year) (see for example, Blatt and others, 1980).
It is the net sedimentation (sedimentation +/- erosion), compactional subsidence, tectonic subsidence, and rates of base level (sea level, river stage, etc.) change that results in a preserved thickness of rock strata. Hence, although fossil tree stumps themselves represent rapid sedimentation, the rocks above the trees generally represent much longer durations of sedimentation.
See Photographs of Standing Fossil Tree Trunks