CHAPTER 5

RADIOCARBON AGE OF A NEWLY IDENTIFIED SLUMGULLION LANDSLIDE DEPOSIT

by Alan F. Chleborad

Introduction

A newly identified landslide deposit covering an area of more than 100,000 m² immediately adjacent to an upper part of the Slumgullion landslide (fig. 1) was studied to determine its origin, age, and relationship to the Slumgullion landslide. The deposit was mapped, surface features and materials were examined and described, stratigraphic relationships were noted, and samples of wood buried in the deposit were collected for radiocarbon dating (Chleborad, 1993). The surface of the newly identified landslide deposit is composed of fine-grained, altered volcanic debris and scattered and locally concentrated fragments and blocks of volcanic rock. The overall appearance of the deposit is similar to other parts of the Slumgullion landslide that have been characterized as older and inactive. Figure 2 shows the location of the newly identified deposit with respect to previously recognized boundaries of the inactive and active parts of the Slumgullion landslide. The location, orientation, and composition of the deposit indicate that it is part of a landslide that originated in the Slumgullion landslide source area. Comparison of the location of the deposit with a published reconstruction of the pre-Slumgullion topography suggests that a large volume of landslide debris that originated near the present 250-m-high main scarp filled the upper valley below the main scarp and spilled over a 60-m-high, east-west trending ridge (divide). The spillover (newly identified landslide deposit) moved in a southwesterly direction down a ravine in the drainage basin of Slumgullion Creek. Subsequent mass movement of the valley fill within the previously recognized bounds of the Slumgullion landslide truncated the original deposit, leaving a steep scarp and exposing a basal contact with underlying colluvium (fig. 2). The apparent sensitivity of the movement of the newly identified landslide debris to the existing topography suggests that it was emplaced as a relatively slow moving landslide rather than as a catastrophic failure that overran the topography. Based on initial results of the study, it was concluded (Chleborad, 1993) that the newly identified landslide deposit was emplaced as part of a major, early episode of Slumgullion landsliding, and truncation of the original deposit was the result of one or more later episodes of Slumgullion landsliding. All this predates the occurrence of the currently active part of the Slumgullion landslide (fig. 2). Results of a subsequent examination of the degree of pedologic soil development relative to other parts of the Slumgullion landslide (Madole, this volume) further supports this conclusion. Accurate age dating of the newly identified deposit is crucial to an understanding of the early history and chronology of Slumgullion landsliding.

Radiocarbon Age of the Newly Identified Deposit

Wood found buried in clay-rich landslide debris at the distal and truncated ends of the deposit (locations AC-2, AC-4, AC-5, and AC-6 in fig. 2) yielded conventional radiocarbon ages ranging from 1,130 to 1,630 yr B.P. (table 1). At locations AC-2 and AC-6, near the distal end, samples were taken from each of two partly buried logs (partly exposed by erosion) located several meters above the base of the deposit (fig. 3). Wood samples AC-2 and AC-6 were identified as Abies (fir) and Pinus (pine), respectively (Paleo Research Laboratories, unpub. report, 1994). The logs were not extensively decayed allowing sampling and dating of the outer 10 to 20 rings. Partial excavation revealed tree bark and branches on outer parts of the logs indicating that few, if any, of the external rings were missing as a result of decay or erosion. At locations AC-4 and AC-5, at the truncated end of the deposit, wood fragments (maximum dimension approximately 0.2 m) concentrated in a zone at the basal contact with the pre-slide surface were also sampled for dating. Calibrated ±2 ranges for the radiocarbon ages of the samples (table 1) were obtained by applying the tree-ring calibration curves of Stuiver and Reimer (1993).

Conclusions

The close agreement in age of the sampled logs from the distal end of the deposit suggests that a single event (the landsliding) may have caused the death of the trees. The radiocarbon ages indicate that the landsliding probably occurred between 880 and 1,330 years ago (relative to A.D. 1994), assuming that an insignificant amount of time elapsed between the death of the trees and burial. The indicated maximum age for wood collected at the truncated end of the deposit (1,630±50 ¹⁴C yr B.P.) corresponds to a calibrated age of 1,395 to 1,740 years (relative to A.D. 1994), which is interpreted as the probable maximum age for the deposit at that location. The time of occurrence of the now inactive part of the Slumgullion landslide has been reported as about 700 yr ago (Crandell and Varnes, 1960, 1961). Based on initial results of the study (Chleborad, 1993) and the radiocarbon ages shown in table 1, I conclude that Slumgullion landsliding involving a significant volume of landslide debris probably occurred as much as several hundred years prior to that time.

References Cited

Chleborad, A.F., 1993, Description, origin, and implications of a newly identified Slumgullion landslide deposit, San Juan Mountains, southwestern Colorado: U.S. Geological Survey Open-File Report 93-548, 17 p.

Stuiver, M., and Reimer, P.J., 1993, Extended ¹⁴C data base and revised calib 3.0 ¹⁴C age calibration program: Radiocarbon, v. 35, p. 215-230.

Crandell, D.R., and Varnes, D.J., 1960, Slumgullion earthflow and earth slide near Lake City, Colorado [abs.]: Geological Society of America Bulletin, v. 71, no. 12, pt. 2, p. 1846.

______Movement of the Slumgullion earthflow near Lake City, Colorado, in Short Papers in the Geologic and Hydrologic sciences: U.S. Geological Survey Professional Paper 424-B, p. B136-B139.

Parise, M., and Guzzi, R., 1992, Volume and shape of the active and inactive parts of the Slumgullion landslide, Hinsdale County, Colorado: U.S. Geological Survey Open-File Report 92-216, 29 p.

 TABLE 1--Radiocarbon ages and corresponding calibrated ages in years before present of wood from newly identified Slumgullion landslide deposit.

Site	Material sampled	Laboratory sample no.	Conventional1 radiocarbon age ± 1 (14 C yr B.P.)	13C	Calibrated2 age at ± 2 (yr B.P.)
Distal end of deposit (AC-2)	Log (outer 10-20 rings) rings)	Beta-63502	1130±60	-21.9	1270-835
Distal end of deposit (AC-6)	Log (outer 10-20 rings)	Beta-67265	1180±70	-20.6	1290-920
Truncated end of deposit (AC-5)	Wood fragments (relation to bark layer unknown)	Beta-67264	1490±60	-22.5	1495-1295
Truncated end of deposit (AC-4)	Wood fragments (relation to bark layer unknown)	Beta-65145	1630±50	-22.5	1700-1350

¹ Age calculations made with half-life of radiocarbon taken as 5,568 years; ± 1 represents the error from counting of the modern standard, background, and the sample being analyzed. All samples were corrected for isotopic fractionation (¹³C values).

²Radiocarbon ages were converted to calendar years using the tree-ring based calibration program CALIB 3.0 (Stuiver and Reimer, 1993). Calibration used bidecadal data set and a laboratory error multiplier of 1.5 was used to account for analytical uncertainties not accounted for in the conventional radiocarbon ages.