UWIRESEARCHDAY2006-JANUARY26&27,2006 Boulders on the Beach.pdfevidence for palaeotsunami events on...
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The Boulders Our investigations at Galina showed that the storm/tsunami- generated boulders are scattered over the raised coral-reef platform in a broad swathe extending for some one and a half kilometres along, and some way back from the coastal cliff, which varies in height from about one metre to some eight metres above sea level. The largest of the boulders measured nearly 6 metres in length, and was estimated to weigh over 100 tonnes, although most examples were much smaller, in the one to twenty tonne range. The boulders were made of hard limestone, mainly containing the fossilized remains of elkhorn corals. Today, elkhorn coral grows most profusely at the crest of the modern reefs. However, these boulders were evidently not from any modern reef, as the rock forming the boulders was dense and well lithified. They probably originated from near the crest of a fossil coral reef. Now, the modern platform on which the boulders rest is composed of rock that had originated as an ancient coral reef, that has since been raised above sea level. These boulders, in fact, closely resembled those parts of the platform nearest to the sea. It seemed likely, therefore, that they had been torn from the seaward edge of the platform and rolled across it to finally rest further inland. The two largest, 100 tonne boulders lie at 40 and 75 metres from the shoreline, and nearly all the rest of them are found between 80 and 180 metres from the shore. In fact, only three of the 180 boulders measured were less than 30 metres from the shoreline. Many of the blocks had plants, even shrubs growing on them, suggesting they had been there for some time. Observations at other localities include the largest boulder seen so far, at Manchioneal, estimated at more than 200 tonnes. Storm-emplaced boulders at Galina Point, St. Mary 200 tonne boulder at Manchioneal Giant waves transporting large boulders Storm Surge or Tsunami? Geological evidence for past, and historically undocumented giant wave events, in the form of boulder fields and coastal debris ridges, has been recorded from six locations on the north, east and southwest coastlines of Jamaica. The sites occur on raised Pleistocene reef terraces, at heights of up to 10 m above sea level and face zones where deep water extends relatively close inshore. Most lack the protection afforded by modern reef development. Boulder lithologies closely resemble that of the platforms on which they rest, and they appear to have been torn from the front of the platforms and transported to their present locations by giant waves. The waves may have been caused by storm surge and waves from very powerful hurricanes, or from the large waves generated by tsunami (often, and incorrectly called tidal waves). Limestone platform with boulders, Negril BIG BOULDERS AT THE BEACH The Marine Geology Unit U UW WI I R RE ES SE EA AR RC CH H D D A A Y Y 2 20 00 06 6 - - J J A AN NU U A AR R Y Y 2 26 6 & & 2 27 7, , 2 20 00 06 6 Debris Ridges At Galina the boulder field is backed backed by a low vegetated ridge consisting of unsorted coral debris, sand, blocks and small boulders. Although this is the only ridge studied so far in any detail, we have observed similar ridges near Discovery Bay and along the south coast, between Negril and Savanna-la-Mar. A pilot study carried out at two localities on Grand Cayman indicated that the size ranges, distribution patterns and physical environments of the boulders there are similar to those observed in Jamaica. Debris ridge at Discovery Bay Debris ridge at Galina Point Debris ridge at Pedro, Grand Cayman The Marine Geology Unit Department of Geography and Geology University of the West Indies Mona Campus Kingston 7, Jamaica Tel: (876) 927-2728, (876) 977-1441 Fax: (876) 977-6029 Email: [email protected] http://www.uwimona.edu.jm/geoggeol/MGU/ Beach Erosion And Coastal Hazards: Ensuring Safety Evaluation of Coastline Changes and Hazards in Jamaica to Improve Hazard Reduction Strategies for Coastal Communities Environmental Foundation of Jamaica Project No: 03/09/431 - PL480 References Robinson, E., Rowe, D.-A. C. and Khan, S.A. 2005a. Mystery boulders at Galina Point. The Gleaner for October 13, 2005. Robinson, E., Rowe, D.-A.C. and Khan, S.A. 2005b. Geological evidence for palaeotsunami events on the coast of Jamaica. Geological Society of Jamaica 50 th Anniversary Conference, December 1-4, 2005, Programme, Abstracts and Field Guide, p. 16. Rowe, D.-A.C., 2005. Boulders on Caymanian shorelines. Internal Report to Marine Geology Unit, Department of Geography and Geology, UWI Mona. Campus. Scott’s Cove Hogg’s Haven Discovery Bay Boulder localities THE MARINE GEOLOGY UNIT UWI, MONA
Transcript of UWIRESEARCHDAY2006-JANUARY26&27,2006 Boulders on the Beach.pdfevidence for palaeotsunami events on...
The BouldersOur investigations at Galina showed that the storm/tsunami-generated boulders are scattered over the raised coral-reefplatform in a broad swathe extending for some one and a halfkilometres along, and some way back from the coastal cliff,which varies in height from about one metre to some eightmetres above sea level. The largest of the boulders measurednearly 6 metres in length, and was estimated to weigh over 100tonnes, although most examples were much smaller, in the oneto twenty tonne range. The boulders were made of hardlimestone, mainly containing the fossilized remains of elkhorncorals. Today, elkhorn coral grows most profusely at the crestof the modern reefs. However, these boulders were evidentlynot from any modern reef, as the rock forming the boulders wasdense and well lithified. They probably originated from near thecrest of a fossil coral reef. Now, the modern platform on whichthe boulders rest is composed of rock that had originated as anancient coral reef, that has since been raised above sea level.These boulders, in fact, closely resembled those parts of theplatform nearest to the sea. It seemed likely, therefore, thatthey had been torn from the seaward edge of the platform androlled across it to finally rest further inland. The two largest,100 tonne boulders lie at 40 and 75 metres from the shoreline,and nearly all the rest of them are found between 80 and 180metres from the shore. In fact, only three of the 180 bouldersmeasured were less than 30 metres from the shoreline. Many ofthe blocks had plants, even shrubs growing on them, suggestingthey had been there for some time. Observations at otherlocalities include the largest boulder seen so far, at Manchioneal,estimated at more than 200 tonnes.
Storm-emplaced boulders at Galina Point, St. Mary
200 tonne boulder at Manchioneal
Giant waves transporting large boulders
Storm Surge or Tsunami?Geological evidence for past, and historically undocumented giant waveevents, in the form of boulder fields and coastal debris ridges, has beenrecorded from six locations on the north, east and southwest coastlinesof Jamaica. The sites occur on raised Pleistocene reef terraces, atheights of up to 10 m above sea level and face zones where deep waterextends relatively close inshore. Most lack the protection afforded bymodern reef development. Boulder lithologies closely resemble that ofthe platforms on which they rest, and they appear to have been tornfrom the front of the platforms and transported to their presentlocations by giant waves. The waves may have been caused by stormsurge and waves from very powerful hurricanes, or from the large wavesgenerated by tsunami (often, and incorrectly called tidal waves). Limestone platform with boulders, Negril
BIG BOULDERS AT THE BEACHThe Marine Geology Unit
UUWWII RREESSEEAARRCCHH DDAAYY 22000066 -- JJAANNUUAARRYY 2266 && 2277,, 22000066
Debris RidgesAt Galina the boulder field is backed backed by a low vegetated ridgeconsisting of unsorted coral debris, sand, blocks and small boulders.Although this is the only ridge studied so far in any detail, we haveobserved similar ridges near Discovery Bay and along the south coast,between Negril and Savanna-la-Mar.
A pilot study carried out at two localities on Grand Cayman indicatedthat the size ranges, distribution patterns and physical environmentsof the boulders there are similar to those observed in Jamaica. Debris ridge at Discovery BayDebris ridge at Galina Point Debris ridge at Pedro, Grand Cayman
The Marine Geology UnitDepartment of Geography and GeologyUniversity of the West IndiesMona CampusKingston 7, JamaicaTel: (876) 927-2728, (876) 977-1441Fax: (876) 977-6029Email: [email protected]://www.uwimona.edu.jm/geoggeol/MGU/
Beach Erosion And Coastal Hazards: Ensuring SafetyEvaluation of Coastline Changes and Hazards
in Jamaica to Improve Hazard Reduction Strategiesfor Coastal Communities
Environmental Foundation of Jamaica Project No: 03/09/431 - PL480
ReferencesRobinson, E., Rowe, D.-A. C. and Khan, S.A. 2005a. Mysteryboulders at Galina Point. The Gleaner for October 13, 2005.
Robinson, E., Rowe, D.-A.C. and Khan, S.A. 2005b. Geologicalevidence for palaeotsunami events on the coast of Jamaica.Geological Society of Jamaica 50th Anniversary Conference,December 1-4, 2005, Programme, Abstracts and Field Guide, p. 16.
Rowe, D.-A.C., 2005. Boulders on Caymanian shorelines. InternalReport to Marine Geology Unit, Department of Geography andGeology, UWI Mona. Campus.
Scott’s Cove Hogg’s HavenDiscovery Bay
Boulder localities
THE MARINE GEOLOGY UNITUWI, MONA
