Time capsules: the use of caves to infer landscape evolution.

Sean A. Stasio

May 12, 2003

Abstract. Presented here is a review of evolutionary landscape development as

inferred from caves. Both historical and present research is examined, as well as dating

techniques. Topics include the establishment of a relation between cave systems and the

surrounding landscape, the importance of context in interpretation, and the benefits of

different dating techniques.

Introduction

Global climate change has severe implications for humanity. Of the many

considerations are the different geomorphic responses to global warming. What will

happen? Will we have more flooding, falling water tables, increased sediment input to

reservoirs? To answer these questions geomorphologists and geologists must have a

reference from which to work---a baseline.

By looking into past geomorphic responses to climate change, a model can be

built that, when applied to future climate change, can tell us something of the future.

Society can use these historical reconstruction models as analogies.

Caves play a large part in the reconstruction of landscape evolution by providing

an undisturbed record of past changes. Held within their passages are deposits which tell

us of river incision and aggradation, uplift response to increased erosion, and general

landscape response to glaciation, just to name a few.

In order for caves to be valuable in the reconstruction of landscape evolution, they

must be related to local geomorphological features like river and marine terraces. If that

relationship can be established, and the ages of the caves or sediment within the caves

dated, then trends and rates of change can be inferred. The idea of inferring landscape

evolution from cave morphology and sediments is the subject of this review.

Cave levels and the surrounding landscape

The use of caves in the reconstruction of landscape evolutionary history relies on

a firm connection between the history of the cave system in question and the surrounding

landscape. As base level changes, due to river degradation or aggradation, sea level

change, or uplift, horizontal cave passages can be left above the zone of phreatic erosion.

These passages may hold sediment deposits, left by the retreating waters, back flooding,

or current stream flow. These sediments, along with speleothems that establish

themselves after phreatic conditions, provide the material that can be dated. If the

passages that house the sediments can be accurately related to the local landscape, then

the sequence and timing of historical geomorphic events can be inferred. Due to the

importance of establishing the relationship of the cave system to the local landscape it is

necessary to review the previous work on this subject.

The idea of relating cave formation to landscape evolution has gained the

attention of many geomorphologists since the first half of the 20th century. Many writers

credit Sweeting (1950) as the first to realize the genetic relationship between caves and

river terraces (White & White 1974, Palmer 1987, Pease et al. 1994), although Droppa

(1966) mentions important work by Calembert in 1950 and 1952. White and White point

out that Davies (1953, 1957, 1960) incorporated the idea of base level control within his

shallow phreatic model for cave development (1974).

Sweeting studied the caverns in the English district of Ingelborough, where he

proposed the creation of caves at distinct levels to be the result of stable water table levels

(1950). The stability of the water table lead to lateral erosion of the cave passages

whereas rapid drops in base level caused by rejuvenation created steep passages and

“pitches”. The lowering of the water table left the previously formed passages “above

the zone of maximum erosion” (Sweeting 1950). The rapid draining of the upper

passages may have also led to the formation of large caverns through roof collapse.

Sweeting also noted that rejuvenation had a greater impact on the western portion of the

Ingelborough district due to its proximity to the base level. This is a principle that will be

expanded on by later authors.

Sweeting again found a relationship between the Buchan caves of Australia and

the Buchan and Murrindal rivers (1960). Sweeting based his analysis on the close

association of river terraces with the location of the Buchan caves. Furthermore, he

reports that the caves follow the same trend as the valley, which would not be the case if

they had developed under phreatic conditions. Sweeting uses the relation of the caves to

the river terraces to conclude that the caves are of Pleistocene origin. This idea falls

neatly in line with evolutionary schemes posed for the Northern Hemisphere that most

geomorphologic features are assigned to Pleistocene origin with formation due to glacial

activity and sea level change. However, Webb et al. researched the same area and

concluded that the terraces and caves of the Buchan area were far older than the

Pleistocene, dating to at least 730 ka (1992). The work of Sweeting on the relationship

between caves and the surrounding landscape, while mostly observational, was a valuable

contribution to landscape evolutionary studies.

(Miotke and Palmer 1972)

Later studies, done by Droppa (1966), Miotke and Palmer (1972), and White and

White (1974) further solidify the concept of cave level and river terrace correlation.

Droppa (1966) and Miotke and Palmer (1972) give similar evidence for the support of

this idea:

1. The elevations of cave passages agree closely with river terraces.

2. Some of the fluvial sediments correspond.

3. Similar gradients for both surface rivers and cave passages.

4. Cave passage and river direction similar.

5. Elevations of cave passages not influenced by stratigraphy or geologic structure,

though they do influence the trend and gradient to some extent.

(Palmer 1987)

Both studies found good correlation between the relationships of cave levels to nearby

river terraces, although Droppa (1966) cautions workers not to assume the same

relationship for all caves. Palmer suggests that greater attention must be paid to cave

surveying to distinguish true cave levels and to firmly establish the conditions under

which the cave passages formed (1987).

White and White used cave records and surveys for the entire Potomac River

Basin to investigate the idea that river incision can be inferred from cave levels (1974).

Their goal was to reexamine the validity of Davies’ model for cave genesis. They found

a strong correlation of horizontal passages with stream levels, although they note that this

pairing is best represented in the downstream portions of the Potomac River Basin. The

headwater regions appear to be somewhat buffered by distance from the basin outlet and

consequently, the local rivers and streams are more related to local conditions.

(White and White 1974)

As noted by several authors (Sweeting 1950, White and White 1974, Johnson and

Gomez 1994) the geomorphic response of caves to base level lowering can depend on the

distance from the dominant base level. In other words, local base levels have more of an

effect on cave levels than that of major rivers and falling sea levels. White and White

contribute this to valley uplands that are more mature and thus better able to handle

changes in base level (1974).

A relatively recent study proved this point succinctly. Johnson and Gomez

showed that the full impact of base level lowering propagates slowly throughout the

entire basin (1994). This occurs because localized readjustments can be absorbed by both

subsurface and surface drainages by “valley headwall retreat and incremental

concentration of flow through master conduits” (Johnson and Gomez 1994). By these

processes the localized lowering of the piezometric was responsible for the multi-tiered

caves in their study, not the incision of the surface stream.

The study by Johnson and Gomez, as well as the caveats noted by other authors

reinforces Palmer’s contention that strict surveying and investigation is needed before

cave levels can be associated with surface streams (1987). If this step is taken with

diligence, and the caves relation to surface features proven, then further analysis can

commence, “with no reason to test the hypothesis of base-level control any further, future

studies should concentrate on regional correlation and on interpreting the history of base-

level changes, past climates, erosion and deposition, and paleohydrology” (Palmer 1987).

Dating

As discussed previously, researchers can relate the elevation of cave levels to

river terraces (and marine terraces?). From this relationship the timing and sequence of

landscape evolution can be inferred. However, in order for this work to be accomplished,

the age of the different cave levels must be determined. No research technique has come

to light that actually dates the caves themselves. Instead, workers must rely on deposits

within the caves. Several techniques are available for the dating of these deposits. The

most useful are uranium series, electron spin resonance and thermoluminescence,

magnetostratigraphy, and cosmogenic 26Al and 10Be dating (White 1988). Other dating

techniques exist, i.e. carbon-14, radium, and amino acid Racemization dating, but they

are of limited use due to age constraints and/ or scant datable deposits. A summary of the

four dating techniques is presented below.

Uranium Series

Uranium series dating is based on the radioactive decay of 238U to 206Pb. An

intermediate step in this sequence is 234U, which has a half-life of 2.45 x 105 years before

it decays to 230Th. Uranium series dating is possible due to the fact that thorium is

insoluble in water and is essentially not found in groundwater; any thorium present is the

result of 234U decay. A determination of the ration between the decay rates of different

isotopes allows for three different dating calculations: 230Th/234U, 234U/238U

disequilibrium, and 231Pa/235U. Use of 234U/238U disequilibrium is not very common due

to difficulties in knowing the initial ratio of 234U/238U before decay and the protactinium

method is also problematic because of difficult processing steps. The most commonly

used method is the 230Th/234U dating calculation, which has an effective age range of

350,000 years. The uranium series techniques can only be applied to the dating of

speleothems. The criteria for a datable speleothem are that it contain at least 0.1 ppm of

uranium, no detrital thorium, and has not undergone any recrystallization (White 1988).

Electron Spin Resonance and Thermoluminescence

The radioactive decay of uranium, thorium and other daughter isotopes in

crystalline solids caused by their own internal radiation fields produces holes in which

electrons become trapped. Electrons will continue accumulate in these traps until the

necessary amount of energy is present to release the trapped electrons. The methods of

measuring the amount of trapped electrons are called electron spin resonance

spectroscopy and thermoluminescence spectroscopy. ESR measures the unpaired spins

of the trapped electrons by training an applied magnetic field against a fixed-frequency

microwave signal. By measuring the power loss, or changes in absorption of the

microwave signal, the number of unpaired spins can be detected. If the radiation field

that produced the trapped charges can be estimated, then the length of time the charges

have been trapped will be known. By merely detecting the trapped electron, and not

evicting them, ESR has an advantage over thermoluminescence in that the samples are

not destroyed and can be repeatedly dated (White 1988).

In contrast, thermoluminescence spectroscopy works by releasing the trapped

electrons by applying the necessary amount of energy. As the charges are freed, they

produce a pulse of light known as thermoluminescence. The age of the sample can be

determined by measuring the intensity of light that is released. Both techniques can

establish dates past the Brunhes-Matuyana magnetic reversal at 730,000 years. This

reversal can be used to calibrate the dates, as can uranium series dating.

Magnetostratigraphy

Magnetostratigraphy works by measuring the remnant magnetic orientation of

magnetic minerals at the time of deposition. This dating technique relies on the reversals

of the magnetic poles to establish discrete time sequences, some lasting hundreds of

thousands of years, which are known from basalt ridges in the mid-Atlantic. An exact,

but course time frame can be established by comparing the local magnetic reversal

sequence recorded in the deposited sediment to the basaltic record (White 1988). The

great advantage of magnetic dating is the ability to see several million years into the past

(Schmidt 1982). For example, any sediment with a reversed magnetic orientation can be

related to a minimum age of 730,000 years, the Brunhes-Matuyana magnetic reversal.

The disadvantage is that portions of the magnetic sequence might not have been recorded.

Sediment with a magnetic reversal could have been deposited after the Brunhes-

Matuyana magnetic reversal, or it might have been deposited after the end of the Olduvai

normal, at 1.87 million years ago. Careful consideration of the local landscape history,

along with comparisons of other dating techniques, will need to be taken into account.

Cosmogenic

Cosmogenic dating is the process by which the ratio of decaying 26Al to 10Be can

be measured to determine the age of quartz particles. The bombardment by cosmic ray

neutrons and muons of quartz near the surface of the lithosphere produces 26Al to 10Be by

spallation of O and Si found in (Granger2001). The production of 26Al and 10Be isotopes

continues at an estimated rate until the quartz particles are shielded from bombardment,

usually by burial. At the time of burial, the process of radioactive decay begins. The

isotope 26Al decays at a faster rate than 10Be, which decreases the ratio of 26Al/10Be

exponentially (Granger 2001). Models of 26Al to 10Be production on the surface allow

for the initial concentration to be known. As long as burial time has exceeded the half-

life of 26Al, the age of the buried sediments can be determined. Cosmogenic dating offers

exciting new possibilities in geologic and geomorphic research due to its accuracy and

range.

Studies

The dating of speleothems and cave sediments provides a basis for the age and

sequence of geomorphic processes on the surface. Both materials can date the timing of

abandonment of the horizontal passages within cave systems. Correlation with the

surface river can then offer a timing of river incision, aggradation, or uplift. Though

problems still remain, i.e. extreme flood events, recrystalized or contaminated

speleothems, these features still provide the best dating possibilities within cave systems.

Speleothems

Speleothems are a collection of formations created by the chemical deposition of

calcite and other minerals (White 1988). Vadose conditions are needed for CO2 diffusion

from water and the formation of speleothems (Jennings 1985). Because speleothems

form under vadose conditions, they can provide a good estimate for the time of base level

drop. Several criteria are necessary for the dating of speleothems; that no change has

occurred since formation, and that the speleothems are found in situ. Methods for dating

speleothems are uranium-series and ESR/thermoluminescence, though carbon-14 can

also be used.

Williams (1982) had success using uranium series dating on speleothems to study

uplift rates in New Zealand. The dated speleothems were located in caves that are part of

a limestone layer that is related to marine terraces. The caves were formed in association

with past sea levels and by dating the speleothems that formed after abandonment by

active streams an age was determined for the cave level. Successive dates at different

levels yielded an uplift rate. Two different uplift rates were calculated from separate

localities, which suggest differential uplift rates for this area of New Zealand.

(Williams 1982)

Gascoyne et al. used similar methods in England to investigate the landscape

evolution of the Yorkshire Dales. They found valley entrenchment rates of 5 to

10cm/Ka, which puts the formation of the Yorkshire Dales between 1 and 2 million years

ago. Their results indicate that glaciation had limited effects on erosion in the Yorkshire

Dales.

A reexamination of the Buchan Karst in Australia by Webb et al. also looked at

the effects of glaciation on landscape development (1992). The use of uranium series and

magnetic dating gave ages that were far older than previously assigned to area (Sweeting

1960). The research of Webb et al. proves that all cave levels date beyond the Brunhes-

Matuyama reversal making them all older than 730 ka, while Sweeting had assigned

Pleistocene age to this area (Sweeting 1960). Furthermore, these dates infer only 2-3 m

of incision over this time period. This illustrates the minor effects that glaciation had on

this landscape.

(Farrant et al. 1995)

Not all dating studies using speleothems prove successful. Farrant et al. found

that most of the speleothem ages in the limestone caves in Sarawak, Malaysia went

beyond the effective age range of both uranium series and ESR techniques. They were

able to date three samples to determine a base level lowering rate of 0.19 m/Ka. To

confirm the speleothem dates, and to extend the dating beyond the range of uranium

series and ESR dating, the authors turned to magnetic dating. The results were

comparable to those obtained earlier, 0.19 m/Ka. However, a major purpose of their

study was the identification of short-term variations in base level lowering. A major

problem with magnetic dating is the course time frame. The solution was to compare the

elevation of wall notches within the caves, formed during periods of aggradation, to

deep-sea records of glacial activity. If base level lowering were constant, then there

should be a correlation between the elevation of wall notches and the deep-sea record.

They found that base level lowering has remained constant over the last 700 ka. This

proved that aggradation was not the result of uplift. They conclude that, “uplift rates are

controlled by isostatic adjustments occurring in response to regional denudation dominate

landform development” (Farrant et al.)

Sediments

Clastic Cave sediments are classified into two types: autochthonous and

allochthonous. Autochthonous sediments are those produced within the cave system,

while allochthonous sediments are transported from the surface (White 1988). Examples

of autochthonous sediments are weathering detritus and breakdown. While both are

helpful in cave geomorphology, neither can infer landscape evolution. Allochthonous

sediments can enter caves through gravitational forces or transportation by fluvial, glacial

and aeolian processes (White 1988). Sediments transported by fluvial action into cave

systems are useful in the reconstruction of geomorphic events because they provide an

undisturbed record that is datable with magnetic and cosmogenic techniques.

The dating of cave sediments can be divided into two approaches:

magnetostratigraphy and cosmogenic 26Al to 10Be. While both approaches are very

different they compliment each other because of comparable age ranges; one could be

used to calibrate or validate the other. Also, together they provide a better chance of

dating material within the sediment because not all cave sediments will have quartz or

magnetic minerals in their makeup.

Magnetic

(Schmidt 1982)

Schmidt’s examination of sediments within

Mammoth Caves was the first to use

magnetostratigraphic-dating techniques for the

study of landscape evolution (1982). Earlier

studies had examined local events and their

correlation with regional curves, whereas

Schmidt’s research was concerned with dating the

origin of the Mammoth caves. The ages inferred

from the sampled sediments suggest that the

Mammoth Caves are at least 900,000 years old, and

could be closer to 2 million years. The difficulty in

establishing a more definitive age is illustrative of

the problem with magnetic dating. Dates provided

by magnetostratigraphy are very precise, but rather

course. For example, clastic sediments at the 11.5m level in the Punchbowl-Signature-

Dogleg cave system show a reversed polarity and are interpreted to date to the Brunhes-

Matuyana magnetic reversal at 730,000 years. The sediments above this level also show

reversed polarity, however, a weak normal polarities were also found at 12.5m. Schmidt

et al. present two interpretations. The first suggests that the all the reversed samples are

assigned to the Matuyama period, making the whole cave system younger than 1 Ma in

age. The other is to consider the weak normal polarity at 12.5m as the Jaramillo normal

with a mixed polarity signal at 25m as the Olduvai reversal (1.67 Ma). This would place

the oldest sediments at 2.3 Ma. After determining the incision rates for each scenario,

(the first would require an unsupported dramatic decrease in local incision rate) the

authors chose the latter of the two options (Schmidt 1984). This study illustrates the

importance of coupling the magnetic dates with the local geomorphic history.

Schmidt, along with Sasowsky and White, successfully applied magnetic dating to

the investigation of stream incision rates in the Cumberland Plateau (1995). Their

findings suggest an incision rate of 0.006 m/Ka. Based on an assumed constant incision

rate they placed the initial incision into the Cumberland Plateau in the early Pliocene (4.6

Ma).

Later efforts at magnetic dating focused on correlation of different cave systems,

the timing and sequence of different cave systems to base level lowering, the use of more

precise sediment samples, and new efforts to constrain ages. Pease et al. compared the

magnetic orientation of sediments in the caves of the Wyandotte Ridge in southern

Indiana with those investigated by Schmidt in Mammoth Caves, Kentucky (1994). Good

correlation was found between the reversed polarity sediments in the Blue River Group,

Wyandotte Ridge, and the two lower levels, C and D, in Mammoth Caves, which

suggests an age of 730,000 years. The sediment from caves of the Stephensport Group,

Wyandotte Ridge, exhibit a normal polarity, which puts them at one of the normal

periods within the Matuyama Epoch (Jaramillo, Olduvai, or Reunion), or possibly back

into the Gauss Epoch. The authors rejected the younger normal events because they

would imply a much greater incision rates than those inferred form the initial realignment

of Ohio River.

(Pease and Gomez 1994)

Building on this work Pease and Gomez (1997) investigated the geomorphic

responses of Wyandotte Cave to that of the Marengo Cave located further from the Ohio

River. Their contention is that base level drop had less of an impact on the Marengo

Cave system because of its distance from basin outlet and greater maturity of the

surrounding landscape. These findings support work cited earlier by Johnson and Gomez

(1994), Sweeting (1950), and White and White (1974). What is becoming apparent is the

pattern of local relationship between caves and the surrounding landscape. This does not

restrict the ideas presented in this paper to local evolutionary models, just that care must

be taken in attributing findings to larger scale models.

The use of

magnetostratigraphy is based

on several criteria-multi level

cave systems that represent

previous base levels and

sediments associated with base

level. Frequently, as Droppa

pointed out (1966), these

conditions do not exist.

Springer et al. encountered

this problem while

investigating the Cheat River incision rate (1997). Due to the lack of multiple, tiered

caves and questions about the exact location of a river during sedimentation of caves,

Springer et al. created total error boxes that constrain both the elevation of the river to

within 40m of the cave elevation and the dates to the different polarity events. From

these boundaries they were able to construct a range of possible incision rates from 56.0

mm/Ka to 63.2 mm/Ka.

Cosmogenic

The use of cosmogenic 26Al and 10Be dating is a relatively new technique in cave

science. While the process is widely used for other geologic and geomorphic questions,

only Granger has published any studies using cosmogenic dating on cave sediment. This

situation will surely not last. Cosmogenic dating offers accuracy, range (0.3-5 Ma), and

a time scale that spans the present to the distant past. Future research will probably

combine cosmogenic dating with other forms of dating as a means of calibration and

validation.

(Granger et al. 1997)

Granger et al. first presented cosmogenic

26Al and 10Be dating in their examination of

incision rates of the New River in Virginia (1997).

In their analysis they discovered slightly different

incision rates at two locations. They determined

this to be the result of local uplift due to a fault in

the study area, which they claim, would be difficult

to detect by other methods. An average rate of 27.3±4.5 m/m.y. was determined, which

was in near agreement with other derived rates for the area. Granger et al. provide the

first radiometric estimate for incision of the New River and prove the usefulness of

cosmogenic dating (1997).

Cosmogenic dating also proved useful in determining the evolutionary history of

the Green River in Kentucky. Granger et al. define 7 major events that relate to river

incision, stability, and aggradation in the Mammoth Cave and the Green River valley all

related to the advancement and position of ice sheets in the area north of Mammoth Cave.

Due to the precise nature of their findings in relation to other works, I will briefly review

their work:

1. River aggradation and sedimentation at around 3.25 Ma.

2. Following a slow incision of the fill, the river again aggraded filling the upper

caves Mammoth Caves (A & B) with sediment at 2.30 Ma.

3. Rapid incision of the Green River, with subsequent incision in Mammoth Cave, at

around 1.92 Ma.

4. The Green River stabilizes at elevation 197 m (level C in Mammoth Cave).

5. Incision through 15 m of bedrock at around 1.39 Ma, stabilizing again at level D,

or 151 meters above current stream level.

6. At 1.24 Ma the river again incises, causing the abandonment of Mammoth Cave’s

level D.

7. River aggradation, filling the lowest level of Mammoth Cave with 10 meters of

sediment at 0.7to 0.8 Ma.

The work of Granger et al. provides an accurate determination of the incision history

of the Green River. In addition, their analysis of burial dates, erosion rates, and

incision rates suggest that slope erosion rates remained mostly unchanged even as the

Green River continued to incise.

Conclusion

This paper reviews the historical and current research into the use of caves in

landscape evolutionary studies, while also providing a background on dating techniques.

In order for sediments and speleothems in cave systems to be used in the evolutionary

reconstruction of the surrounding landscape, they must first be related to those

landscapes. The importance of context cannot be overstated. Some of the research

reviewed here suggests that not all caves are useful in this endeavor. Other research

shows the disjunct relationship between cave systems and distant base level lowering.

However, if context is carefully examined, the techniques presented here can date major

geomorphic events. These events are river incision and aggradation, tectonic uplift, base

level lowering, sea level rise, and interstitial and interglacial periods. Besides the

historical reconstruction of landscape evolution, these studies can provide a baseline with

which to predict geomorphic response to future changes like global warming. These

studies will not illustrate the exact geomorphic consequences of our actions because they

transpired under very different circumstances. However, the responses in the past can be

instructive, and can give us clues as to what to expect in the future.

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