Geologie

Subaqueous artesian springs

Introduction

The limited fresh water resources of our planet are facing steadily increasing human and industrial consumption, as well as various threats by chemical, radioactive and biological pollution. With increasing difficulties in the exploration of new groundwater resources in future, the knowledge about groundwater processes and spring formation will increase in importance. In this investigation, active under water springs in a Himalayan lake are compared with cylindrical, vertical water channels and spring pits from a Lower Devonian barrier-island environment; both, the active and the fossil example are explained by up-welling artesian ground water.

 

Active, subaqueous, artesian springs in the Lingti Valley:

In the example presented here, up-welling groundwater has produced shallow, circular, some decimetres in diameter large depressions in muddy sediments in a small, high-alpine Himalayan pond. Similar structures were first described from sandy beaches at Maple Lake (Canada) by Quirke (1930), who called them "spring pits". According to Quirke (1930), they formed near the shore-line, both above and below water, by up-welling groundwater after heavy rains which rose with sufficient force to sweep out finer sand grains. Springs are found in a small lake that has formed at the level of the modern floodplain on the left bank of the Lingti River, up-stream of an alluvial fan of a left-side ravine; the lake is some 50 m long and some 10 m wide, with a depth not exceeding c. 40 cm. The lake is interpreted as an abandoned river channel of the Lingti River, dammed by sediments of the alluvial fan. The lake bottom shows thin microbial mats on the surface of the lake mud, which is partly rich in rotting organic material and shows synaeresis cracks in places. The thickness of the mud exposed within the spring pits is c. 30 cm. The sediments below the lake mud are not exposed, but the lake is very small and the subsurface may be reconstructed from the local geological outcrop outside the lake. The northwestern shore of the lake comprises fluvial gravel, the opposite southeastern shore consists of rock debris from nearby cliffs and alluvial fan material at the southwestern termination of the lake.

 

Location map of the investigated locations in Spiti, Himachal Pradesh, NW India. Blue arrow indicates the location of the sub-aqueous springs in the Lingti Valley, the red arrow indicates the location of the Devonian spring pits and sandstone pipes in the Pin Valley.
General view of the small lake in the Lingti Valley; view upstream towards the northeast. Spring pits occur in the right foreground of the photo. In the left foreground, prograding sediments of the alluvial fan are visible. Note the shallowness of the lake.

Spring pits are randomly distributed sub-aqueous depressions in the mud at the bottom of the southwestern part of the lake, in an area within some 5 m of the shore made by the alluvial fan; they have not been found in other parts of the lake. Generally, their shape is circular to slightly elliptical with relatively smooth outlines, but some examples of more irregular shaped spring pits also occur. In several cases two pits have joined to form elongate, elliptical depressions. The pits range in diameter from about 10 cm to 30 cm, although composite pits may reach up to 40 cm in diameter. Spring pits are found in lake mud at some 15 cm water depths where they form depressions up to c. 25 cm deep below the lake water surface. The complete bottom of the depressions is formed by fluidized mud, the sediment being kept in suspension by ascending spring water; solid ground inside the spring pits is reached 45 cm below the lake water surface. Spring pit walls are steep, to nearly vertical, slightly tapering downwards. Usually, the uppermost centimetres narrow downwards at some 50° inclination with relatively even surfaces. Fray out mud from the upper part, rich in microbial activity that increases cohesive forces within the sediment, commonly hangs downwards, partly overhanging steeper parts of the spring pit walls. At the time of our study, all the pits showed spring activity, indicated by fast, irregular movements of fluidized sediment at the spring pit bottoms and trails of mud suspended into the water-column. No gas bubbles ascending from the spring pits have been observed. Additionally, there was no marked difference in temperature between lake water and spring water.

 

Overview of the spring pits close to the shore, formed by the alluvial fan showing their appearance and their distribution on the lake bottom; view towards the north. Springs show broadly similar activities; two spring pits filled with suspended mud are the result of stepping into them before taking the photo. (Lens cap for scale).
Detail of a representative spring pit. Note the appearance of boiling-like moving fluidized sediment at the bottom, the tapering walls and the more cohesive, microbial rich upper part. A smaller, irregular shaped spring pit is seen in the lower right corner. (Lens cap for scale)

Schematic diagram illustrating a conceptual hydrogeologic model of the formation of spring pits. Not to scale. Lake mud and soil above fluvial gravel and alluvial fan material forms a local aquiclude for the groundwater flow in coarse-grained, highly permeable alluvial fan sediments towards the valley axis. These conditions result in a locally increased hydrostatic head that forms circular springs when artesian springs break through the mud.

Fossil, subaqueous, artesian springs in the Muth Formation:

The Early Devonian Muth Formation represents a prominent lithology within the Paleozoic successions of the Higher Himalayan tectonic unit at the northern Indian sub-continent. Except a thin dolomitic horizon in its upper part, the formation consists solely of pure, mature quartz arenites. The Muth Formation has been divided into four facies associations, a barrier-island depositional environment is suggested. Sandstone pipes are found exactly below and within a thindolomitic interval, which is interpreted as lagoonal deposits. Together with other liquidization structures like sand pits, these structures are found at a distinct level that can be correlated across three sections with a total restored distance of 31 km perpendicular to the overall facies trend.
Cylindrical structures, cross-cutting stratification at right angles, are up to 1.5 m in height and 0.8 m in diameter with an internal structure comprising concentric, cylindrical laminae. The pipes, which probably represent water conduits for laminar upward flow of ground water, initiate from relatively thin horizons, with upper terminations formed by spring pits. Thus the structures in the Muth Formation represent a rarely observed combined occurrence of spring pits and their conduits below. Their formation is explained by rising ground water seepage in a coastal depositional environment that produced a relatively high hydrostatic head, resulting in the formation of springs. The rise in relative sea-level might be related to tectonic subsidence caused by tectonic activity linked to the formation of conjugate deformation bands in the Muth Formation. This means, if tectonic activity was involved, it did not form the cylindrical structures by seismic liquefaction directly, but might be responsible indirectly through ground water seepage rise resulting from tectonic subsidence. Due to the little relief in this environment, the sea-level rise affected a relatively large area and fluidization structures can be found widespread in distant sections.

 

Top surface of bed Me440 showing the uppermost parts of several sandstone pipes (Figures 7a&b) weathered into the surface. (Hammer for scale).
Detail of the uppermost part of a sandstone pipe (Figure 6a). Note the slightly raised rim and some thin sand fissures running across.
Sandstone pipe in bed Me440, cross-cutting tangential aeolian foresets. Boundary of the sandstone pipe is very sharp, foresets laminae do not show any deformation outside. (Compass for scale).
Lower bedding surface of bed Me440 shows axial section of a small sandstone pipe with concentric laminae.

Conclusions:

Circular depressions, found at the bottom of a small Himalayan lake represent spring pits (Quirke 1930), formed by active vertical discharge of groundwater from an artesian alluvial fan aquifer, confined by an aquiclude of fine-grained lake sediments. The aquifer is continuously recharged by down slope groundwater flow into the alluvial fan.
Numerous circular depressions found on bedding surfaces in Lower Devonian barrier island arenites from the northwestern Himalayas represent spring pits formed by upward flow of ground water. Spring pits are underlain by concentric sandstone pipes, which are explained as conduits for vertical groundwater flow. These structures thus represent very rare examples of the preservation of fossil spring pits and their conduits below; their formation is explained by variations in ground water seepage in a coastal depositional environment.
Spring pits have commonly been found at the transition of marine/limnic/fluviatile and terrestrial environments, where high water saturation occurs and water table variations are frequent. Thus, spring pits may give indication for palaeo-environmental interpretation in the fossil record.


References:

DRAGANITS, E., GRASEMANN, B. & SCHMID, H.P. (2003): Fluidization pipes and spring pits in a Gondwanan barrier-island environment: Groundwater phenomenon, palaeo-seismicity or a combination of both? In: Van Rensbergen, P., Maltman, A.J. & Morley, C.K. (eds.): Subsurface Sediment Mobilization. Geological Society, London, Special Publications, 216, 109-121. pdf (4.0Mb)

 

DRAGANITS, E. & JANDA, C. (2003): Sub-aqueous artesian springs and associated spring pits in a Himalayan pond. Boreas, 32, 436-442. pdf (0.5Mb)

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