Miocene sea-floor subsidence and later subaerial solution subsidence structures in the Maltese...
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Miocene sea-floor subsidence and later subaerial solution subsidence structures in the Maltese Islands Written discussion of a paper taken as read JOHN NEWBERY The Editor, 16 October 1975 Dear Sir, During a two-year assignment in the Maltese Islands I had occasion to investigate many of the sink hole structures, both by drilling and resistivity, referred to by Pedley (1974). Salient data on my findings have been published (Newbery, 1963; Newbery, 1968), although Pedley was ap- parently unware of this. My investigations were concerned with the Upper Coralline Limestone aquifer where, with one exception, no sink holes had been reported, although the large subsidence structures near the west coast of Gozo and inland were well known. The identification of numerous sink holes in the Upper Coralline Limestone necessitated their subsurface investigation for two engineering reasons: firstly, any proposed drainage galleries would need to be diverted away from sink holes to avoid difficult excavation; secondly, in the graben structures being considered for development as underground reservoirs, sink holes could be a source of leakage from a perched aquifer. Borehole results indicated two modes of origin for the sink holes: either wholesale collapse through a distance of 18 to 60 m into a cave probably formed in the Lower Coralline Limestone, or softening and creep of the Blue Clay formation down solution fissures in the underlying limestone, causing subsidence and brecciation of the overlying Upper Coralline Limestone. In some sink holes the Blue Clay seal which gives rise to the perched upper water table was not broken, an attenuated thickness of clay remaining. In others, brecciated Upper Coralline Limestone is in juxtaposition with G/obigerina Limestone and the puncturing of the clay seal was confirmed by the presence of saline water. To stop leakage through the sink holes would have re- quired highly expensive grout curtains around their periphery. For this and other reasons the proposal to use graben structures as underground reservoirs was abandoned. Detailed comments on Pedley's paper are as follows: 1. Thicknesses of the three youngest formations as proved by drilling are: Upper Coralline Limestone 104 m (max.), Greensand 0-15 m, Blue Clay 0-75 m. 2. The majority of the Upper Coralline sink holes are not separated from the unaffected areas by clearly defined fault planes, although from observations in drainage galleries their loca- tion is considered to be fault-controlled-caves and fissures having formed in brecciated limestone adjacent to a fault plane, generally along the hanging wall. 3. The Ghajn Znuber sink hole, illustrated as fig. 7 in my 1968 paper and called II Mansap, typifies those sink holes considered to have formed by the softening and creep of Blue Clay down solution fissures. REFERENCES NEWBERY, J. 1963. Geological factors controlling water supply in the Upper Coralline Areas of Malta. Ph.D Thesis, University of London. John Newbery Howard Humphreys & Sons Thorncroft Manor, Dorking Road Leatherhead, Surrey KT22 8JB 111 NEWBERY, J. 1968. The perched water table in the Upper Limestone aquifer of Malta. Jour. [nsln Water Engrs. 22, 551-70.
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Transcript of Miocene sea-floor subsidence and later subaerial solution subsidence structures in the Maltese...
Miocene sea-floor subsidence and later subaerialsolution subsidence structures in the Maltese Islands
Written discussion of a paper taken as read
JOHN NEWBERY
The Editor, 16 October 1975
Dear Sir,
During a two-year assignment in the Maltese Islands I had occasion to investigate many of thesink hole structures, both by drilling and resistivity, referred to by Pedley (1974). Salient data onmy findings have been published (Newbery, 1963; Newbery, 1968), although Pedley was apparently unware of this.
My investigations were concerned with the Upper Coralline Limestone aquifer where, with oneexception, no sink holes had been reported, although the large subsidence structures near the westcoast of Gozo and inland were well known. The identification of numerous sink holes in theUpper Coralline Limestone necessitated their subsurface investigation for two engineeringreasons: firstly, any proposed drainage galleries would need to be diverted away from sink holes toavoid difficult excavation; secondly, in the graben structures being considered for development asunderground reservoirs, sink holes could be a source of leakage from a perched aquifer.
Borehole results indicated two modes of origin for the sink holes: either wholesale collapsethrough a distance of 18 to 60 m into a cave probably formed in the Lower Coralline Limestone,or softening and creep of the Blue Clay formation down solution fissures in the underlyinglimestone, causing subsidence and brecciation of the overlying Upper Coralline Limestone. Insome sink holes the Blue Clay seal which gives rise to the perched upper water table was notbroken, an attenuated thickness of clay remaining. In others, brecciated Upper CorallineLimestone is in juxtaposition with G/obigerina Limestone and the puncturing of the clay seal wasconfirmed by the presence of saline water. To stop leakage through the sink holes would have required highly expensive grout curtains around their periphery. For this and other reasons theproposal to use graben structures as underground reservoirs was abandoned.
Detailed comments on Pedley's paper are as follows:1. Thicknesses of the three youngest formations as proved by drilling are: Upper Coralline
Limestone 104 m (max.), Greensand 0-15 m, Blue Clay 0-75 m.2. The majority of the Upper Coralline sink holes are not separated from the unaffected areas
by clearly defined fault planes, although from observations in drainage galleries their location is considered to be fault-controlled-caves and fissures having formed in brecciatedlimestone adjacent to a fault plane, generally along the hanging wall.
3. The Ghajn Znuber sink hole, illustrated as fig. 7 in my 1968 paper and called II Mansap,typifies those sink holes considered to have formed by the softening and creep of Blue Claydown solution fissures.
REFERENCESNEWBERY, J. 1963. Geological factors controlling
water supply in the Upper Coralline Areas of Malta.Ph.D Thesis, University of London.
John NewberyHoward Humphreys & SonsThorncroft Manor, Dorking RoadLeatherhead, Surrey KT22 8JB
111
NEWBERY, J. 1968. The perched water table in theUpper Limestone aquifer of Malta. Jour. [nsln WaterEngrs. 22, 551-70.
