Collapse sinkhole is described as the sudden or progressive collapse of the roof of a cavity into the underlying cavern. Collapse is entirely within karst rocks and can be 100m across and 100m deep.
I am going to examine a sinkhole that I visited couple months ago and try to find an answer of How it’s formed?
The sinkhole, subjected to examination is located in Turkey near Sivas. It is formed in lower Miocene gypsum rocks.
Brief General Geological Setting of the Region
The Sivas region has one of the world’s largest and most mature areas of gypsum karst. (Gunay, 2002; Irfan & Ozakaya 1981; Karacan & Yilmaz 1997 and 2000; Kacaroglu 1997; and Klimchouk, A. et al. 1996) and the Sivas Basin is one of the largest central basins which formed in the collision zone between the pontide and anatolide – tauride belts (Cater et al., 1991; Ciner et al., 2002; Gunay, 2002).
The Sivas Basin was dominated by deep-sea depositional environments during Paleocene and Eocene times (Kurtman, 1973; Kosun and Ciner, 2002) and the marine sedimentation continued until the end of the middle Miocene (Ciner et al., 2002). During the Late Eocene–Oligocene, the basin became an uplifted foreland basin in which several kilometres of clastic-dominated marine sediments with subordinate evaporates accumulated. During the Late Oligocene– Miocene, the basin is considered to have evolved into a locally subsiding lagoonal and molassic basin (Poisson et al., 1996; Ciner et al., 2002).
The gypsum in Sivas Tertiary Basin is present in formations formed in Eocene, Oligocene and Miocene. The gypsum in the subject area is Lower Miocene. It overlies the Oligocen-aged Sandstone / Siltstone & Mudstone unconformably.
The subject area is affected by N-NW converging small thrusts (Ciner et al., 2002). There are faults and folds can be observed around adjacent locations.
Geological and Geomorphological Setting that Affecting the Formation of Collapse Sinkhole
As a result of uplift of the area in the Upper Miocene the River system formed and the basin became an erosional area (Türkmen and Kerey, 1995). The covers over the gypsum were eroded in this period and exposed a higher gypsum plateau. As a result of tectonic activity and continuous eroding by river systems, the rivers incised their valleys deeper. While river systems were eroding the basin, differences in piezometric level play a significant role in the formation of paleovalleys, collapse dolines and poljes.
The subject area stays at the junction of the river systems and the collapsed sinkhole is located at the boundary between polje and plateau karst.
Before examining the formation of collapse sinkhole, it would be better to understand the formation and features of the polje and plateau karst.
After uplift started in the area, piezometric level started to decrease continuously and the surface waters join the underground waters. As a result, the branches of the river became a karstified hanging valleys above the floors of the river valley and formed lower plateau karst.
Mostly this karst has a gently undulating surface with soils that are thin or absent over weathered gypsum. Those uplands are distinguished by isolated large collapse dolines, most of them are ancient features, though some are still active and have lakes on their floors at the altitude of the local water table and nearby river valleys like collapse sinkhole that is subjected to this article.
Poljes are broad, flat-floored, alluviated depressions, typically a kilometre or more across, within the karst (Waltham, 2013). Poljes have formed by corrosional deepening and lateral planation along tectonic and structural lines (Dogan et al., 2005). Typically they are occupied by shallow lakes in the wet season. Streams or underground connection to rivers may drain them.
The polje situated in the subject area is located at the junction of river systems and their floodplains. They provide the base for erosion, and most of its valley is wide and alluvated. The bottom of the polje is below the river floodplain and drained underground. It comprises residual soil and alluvium derived from sand and clay beds interlayered in gypsum unit.
Isolated gypsum hills within the polje areas are likely to contain larger cavities where water is draining or has drained, through the hill from one polje or valley to another.
Poljes enlarge as a result of result of the collapse of cave roofs or the merging or the destruction of collapse dolines.
Formation of the Collapse Sinkhole Situated in the Subject Area
The collapse dolines, characteristic features of the mature karst area of the subject area, show four steps of development (Dogan et al., 2005). The first stage (Fig.1); a collapse of a cavity roof and formation of the collapse doline.
The lowest point of the polje is located at the boundary of polje and plateau karst as it can be seen in the Fig.3. This lowest point is drainage connection point of the temporal lake in polje and river systems.
The polje have seasonally active lake time to time water level increases about 3m above from the lowest point of polje (Fig.4)
By using all these information, chronology of the formation of collapse sinkhole is;
- Deep-sea deposition during Paleocene and Eocene and the marine sedimentation continued until the end of the middle Miocene.
- During the Late Eocene–Oligocene, the basin became an uplifted foreland basin in which several kilometres of clastic-dominated marine sediments with subordinate evaporates accumulated. So, the depositional environment created for gypsum.
- During the Late Oligocene– Miocene, the basin is considered to have evolved into a locally subsiding lagoonal and molassic basin. So the foundation of gypsum rock formation is laid.
- Lithification of the gypsum is completed in Lower Miocene.
- As a result of uplift of the area in the Upper Miocene the River system formed and the basin became an erosional area.
- After uplift started in the area piezometric level started to decrease continuously and the surface waters join the underground waters. As a result, the branches of the river became a karstified hanging valleys above the floors of river valley and formed lower plateau karst.
- By the connection of the river systems and flooding of the rivers, polje formed between plateau karst.
- With underground drainage patterns temporal lake formed in polje bottom.
- Increasing of water level seasonally in temporal lake cause a dissolution on cliffs of gypsum.
- With dissolution of the gypsum, cave have formed at the boundary of polje and plateau karst and enlarged in time
- Due to enlargement of the underground cavity, the roof started to collapse and collapse sinkhole has formed.
After that, dissolution of the gypsum will continue and the cave will continue to enlargement and collapses on the roof will be enlarged. At least one of the side of the doline still maintains its steepness. Then doline reaches its maximum diameter and acquires a circular rim. The final stage involves the retreat of the sides of the doline as a result of dissolution. Fig.5 shows the final stage from another example.
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