Transcript Active Gypsum Karst, Woodville, Codroy Valley, Newfoundland Krista House and Norm Catto, Department of Geography, Memorial University, St.

Active Gypsum Karst, Woodville, Codroy Valley, Newfoundland
Krista House and Norm Catto, Department of Geography,
Memorial University, St. John’s, NL
Scale 1:40,000
Dolines, Woodville area, Codroy Valley, NL
Legend
Green- dolines with dry bottoms
Blue – dolines with standing water
Pink – coalescing dolines
Dashed lines indicate areas with multiple small dolines & depressions
N
Karst dissolution in steeply dipping, faulted gypsum beds has resulted in the development of at least 70
suffusion dolines at Woodville, Codroy Valley, Newfoundland. Gypsum beds are confined laterally by
resistant rock units, focusing and enhancing dissolution. Increases in the amount of precipitation through
climate change, and in the rate at which precipitation enters the groundwater system as a result of
clearing of forest cover, have resulted in increased dissolution. Suffusion dolines, the dominant landform
features at Woodville, have minimum depths of approximately 1-5 m. Differing water levels noted in
adjacent dolines indicates that not all features are interconnected to a common water table. Pre-last
glacial dolines can be seen along the shoreline, thus providing confirmation that this process has been
ongoing for a minimum of 30,000 years.
The 70 dolines in the Woodville area show differing stages of development, although most appear to be
actively propagating and expanding. Linear trends have been identified and predictions can be made as
to where future suffusion dolines will appear. Attempts to infill the dolines have not been successful, and
the addition of waste introduces pollution into the groundwater system.
Development of the suffusion dolines may not be immediately evident on the surface. At
Woodville, the gypsum beds are overlain by 1-2 m of glacial sediment, derived from areas
to the northeast and east. This material is coarse-grained, dominated by gravel and sand,
and does not contain large quantities of sediment derived from the underlying gypsum.
Consequently, it is not subject to dissolution, but instead forms a blanket on the surface
concealing the underlying gypsum units. As the underlying gypsum is dissolved, the
overlying glacial sediment may be sufficiently thick or rigid to permit the formation of
cavities, several of which have been detected through the use of ground-penetrating radar.
In other locations, progressive dissolution of the underlying gypsum causes the surface
blanket to gradually sag into the developing low area.
Failure, producing a sinkhole evident on the surface, occurs when the amount of gypsum
dissolved is sufficient to cause the blanket to ‘tear’. This results in the collapse of the
glacial sediment blanket into the doline, partially filling the bottom with disturbed
material. Dissolution of the gypsum will continue, however, as water infiltrates around
the glacial materials.
Attempts to infill the dolines by
adding sediment, vehicles, and
refuse will not be successful, as
dissolution will continue as long
as either surface or ground
water is in contact with the
gypsum.
Addition of refuse
containing acids or other
chemical
compounds
may
actually act to accelerate gypsum
dissolution, by changing the
chemistry of the water in contact
with the gypsum beds. As well,
the addition of waste into the
sinkholes provides a means to
introduce pollution into the
groundwater system, thus posing
possible danger to community
residents.
Currently, several homes are at risk of damage or destruction from collapse.
Woodville
Gulf of
St.
Lawrence
Preliminary mapping has revealed a minimum of 70 dolines in the Woodville area, each in differing stages of development. Dolines range from hints of depressions in a field to spectacular
holes filled with water. Most appear to be actively propagating and expanding at this time. Several oval dolines have extended laterally to join together to form linear features. Study is
ongoing to determine if the rates of dissolution and doline development have increased over the past 50 years. Properties adjacent to the doline walls are at medium-term risk of damage,
and will eventually suffer collapse as dissolution continues. At the present time, it is not possible to cite a precise numerical rate of dissolution.
Shallow (less than 1.0 m) and small (less than 5 m diameter) depressions are indicative of subsurface
activity. In many areas of Woodville, fields of the features are evident. The proximity of the
depressions to occupied homes raises concerns about the safety of living in certain areas of Woodville.
Ground Penetrating Radar has been used to accurately map the locations of subsurface incipient
suffusion dolines, both to determine their distribution and to try to alleviate some fears concerning
imminent danger to residents. Although in many cases there is no danger of an immediate collapse, the
gypsum underlying the area is continuously being dissolved from beneath, so any sense of security is
temporary at best.
Suffusion dolines are the dominant landform features at Woodville. Exposed dolines have
depths of approximately 1-5 m as seen from the surface, but several that are filled with water
are known to be much deeper. They are circular to oval in plan view, with the some of the
largest boasting a maximum diameter of over 50 m. Walls are generally steep, with modal
slopes typically 60° at the surface increasing to nearly vertical at depth. Water levels at the
bottoms of the dolines vary significantly, depending both on recent precipitation and drainage
through the bases. Differing water levels noted in adjacent dolines indicates that not all
features are interconnected to a common water table
During fieldwork in 2002 and 2003, several active suffusion dolines were identified.
The dolines can be related to underlying structural geology, as illustrated on the map.
The rapidity of dissolution evident throughout 2003 provides ongoing evidence that
significant changes are happening at Woodville. Several new depressions have
appeared since July 2003, indicating that development is occurring much more rapidly
than previously thought. In particular, a small cavity that has emerged on the Samms
property is of significance, as the ground can actually be felt moving at certain times,
and ground-penetrating radar has identified a hollow space beneath the surface. GPR
has been used in several other locales, and has confirmed suspicions that the entire area
is being dissolved from beneath by water in contact with the gypsum. As the gypsum
beds are aligned at steep angles, are cut by faults, and vary in thickness, the dolines are
not forming uniformly over the landscape. Linear trends can be identified and
predictions can be made as to where holes or depressions may appear in the future
based upon knowledge of the bedrock stratigraphy of the area. GPR has helped to add
weight to these predictions, as readings taken along seemingly solid ground identify
hollows underneath that are consistent with the expected procession of the sinkholes.
Woodville has several major active sinkholes that have developed within approximately
the last 50 years, as determined from aerial photography and personal accounts from
residents. Two of the largest holes exist within 50 meters of each other, both showing
signs of recent activity. The bottoms of the holes are generally water filled, but levels
are independent of each other and of precipitation, suggesting that the holes are
connected to different groundwater sources. As can be seen from the photograph
directly below, the two holes are adjacent. Since the photograph was taken in July
2003, a third hole has opened adjacent to the small shed near the center of the photo.
The hole is consistent with the linear trends identified in the fields across the highway.
Development of this doline was predicted, as a depression could be seen at the site in
July. Further testing with GPR has identified additional underground activity
coastward, towards the hill from which the photograph (below) was taken.
The progression of dissolution and
collapse occurring at Woodville
are by no means a new
phenomenon. Evidence for preglacial sinkholes can be seen along
the shoreline, thus providing
confirmation that this process has
been ongoing for more than 20,000
years at minimum.
Dolines
developed in pre-glacial times, and
were subsequently infilled with till
and
other
deposits
during
glaciation and as the ice receded.
The presence of a modern collapse
feature further suggests that the
activity propagates in the same
area repeatedly, utilizing previous
zones of weakness.
Repetitive development and
collapse of suffusion dolines is
evident
throughout
the
landscape.
Developing Suffusion Doline, Holy Trinity Church
Gypsum undergoing active dissolution
The relationship between ground vegetative cover and gypsum dissolution is one that needs to be examined further,
since questions have been raised with regards to whether removal of ground cover for farming or logging leads to an
increase in the speed of karst dissolution. Removal of vegetation would allow more water to infiltrate the topsoil,
resulting in an increase in the rate of gypsum dissolution. Future research will focus on quantification of rates of
dissolution and doline development, and direct assessment of the significance of land use practices.
Typical shallow depressions indicative of underlying suffusion dolines
Ground-penetrating radar surveys in August 2003 revealed numerous subsurface cavities, as
indicated by the high reflectance at depth (left side of image). This suffusion doline, extending to
more than 36 m depth and with a near-surface diameter in excess of 25 m, is typical of those in the
Woodville area. GPR Surveys were conducted by GeoScott Exploration Consultants.
Acknowledgments
We acknowledge the interest and support of NL Environment and Conservation (especially Keith Guzzwell), and NL Natural
Resources (especially Shirley McQuaig). GPR Surveys were performed by GeoScott Exploration Consultants Inc., and we
acknowledge Bill and Sue Scott for valuable discussions.