Beachrocks - their physical and geotechnical characteristics
Ian Wright
A typical beachrock sequences at Exmouth, Australia
If you have been to a sandy beach in the tropics or subtropics, the chances are you have encountered beachrocks.?And no, not every rock on the beach is a beachrock! True Beachrocks are carbonate cemented sandstones that have formed in the Phreatic Zone.?The Phreatic Zone is defined as where the saltwater wedge encounters the freshwater table.?IE typically in the intertidal zone.?Where the chemical conditions are right (temperature, PH, carbonate concentration etc), the carbonate is precipitated out of solution to bind the beach sediments together to form beachrock.
The rocks typically?occur as well graded fine to coarse gravelly sandstones, but may also?be true gravels as well as cobble conglomerates where large particles, particularly coral dominates the sediment.?They tend to have a very shallow seaward dip, which is often enhanced as the seaward face is typically undercut by wave action causing large blocks to slump forward.?Beachrocks are also referred to as calcarenites (after Clark and Walker 1977), caprocks and limestones due to their high carbonate value.
?Beachrocks are very good indicators of periods where sea-level has stood for a number of years and can be found both above and below present sea-level as palaeo-strand (beach) deposits.?Because beachrocks are formed in the phreatic zone they are typically very narrow in the coast normal direction, but continuous in a coast parallel direction.?
?Submerged linear?beachrock sequence, East coast South Africa.??Compliments of Marine GeoSolutions
Beachrock sequences along the Mozambique coastline.?Compliments of Google Earth
Beachrocks may also be associated with aeolianites (fossil lithified dune sequences) and where this occurs, they tend to have a larger topography. Rapid Pleistocene sea-level changes have meant that a number of beachrock sequences (often with aeolianites) may be stacked upon each other or occur as discreet units and this may occur anywhere on the shelf, within 0-200m water depth, but possibly even deeper. Beachrock formation typically excels during sealevel stillstands.?Where beachrocks occur they typically mimic old coastlines and as such tend to form as very long coast parallel barriers which may be kilometres long, but also very narrow in width.?
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?A beachrock/aeolianite sequence from Cabo Del Gardo, Mozambique
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A ‘Pinger” profile showing an aeolianite dune stub on the continental shelf
Where water conditions are ideal, namely warm and clear, the beachrocks often form the base of flourishing ‘coral’ reefs.?In deeper water they may provide the ideal habitat for soft corals and sponges.?
?An in-situ hard coral attached to beachrock indicating that this outcrop has been exposed to shallow marine conditions - this rock is referred to as a conglomeritic calcirudite (but I bet you will never hear one being referened in any geotech work?)
?Geotechnically beachrocks vary from being very weak and friable where cementation has not taken place completely to a hard rock (typically 70 MPa UCS) where carbonate cementation is complete.?This is referred to as a process of induration.?
Discreet beachrocks sequences typically occur as 1m thick units and often occur as "float" in that they may have unconsolidated sediments under them and as such are not true bedrock,?This, of course makes them seismically very difficult to characterise (by both seismic reflection and refraction).?Buried or partially buried beachrock sequences are particularly difficult to resolve geophysically due to their ‘capping’ properties where they provide a dense layer over less dense material.?Their laterally and vertaiclly thin profiles also make them have acoustically similar properties to the surrounding unconsolidated sediments.?Point geotechnical testing (including CPT’s and coring) may not locate buried beachrock sequences due to their restricted coast normal width.?IE they are not laterally continuous sequences.?In other words, from a geotechnical perspective, they are to be treated with the utmost caution.
?Image showing weakly indurated beachrock at the back of the beach grading into more indurated material seaward
#Geophysics, #geotechnical, #geology
Junior Geologist at Kai Batla Minerals Industry Consultants
3 年Great article Ian Wright. Very informative. I am currently looking at the properties that affect their strength. Have you done any 3D tomography on calcarenites?
Does your ground modelling process bring a competitive advantage to your bids? Ask me how.
5 年Hi Ian Wright. Good write up. However, beachrock (or caprock or calcarenite - depending on where you work!) is not necessarily a problem to classify. I agree that legacy ground modelling technologies do not work in these environments.? CPT, DCP are inconclusive due to the highly variable nature of the material. Refraction commonly reports velocity inversions, reflection just bounces off it, or through it and so is also inconclusive and, lets face it, boreholes cannot be used to find the extents of any geological unit. However, we have been using Aquares technology throughout the world to successfully map the thickness density and extents, as well as what is under of beachrocks, for 20 years. In the example below Aquares results are seen on the bottom strip and boomer on the top (the Boomer was hanging out the middle and starboard side and Aquares out the Port side). You can see the thin veneer of sand overlying the beachrock. Also, to the right, the beachrock is thinner and the boomer is "seeing through" it. but the remainder is higher density. Also the blue colours are indicative of washouts below the beach rock. This example happens to be from near Exmouth. Please give me a call if you need any more info on out tech.
Lead Subsea Cable Engineer, Marine Geophysicist & Geospatial Data Scientist at Tetra Tech
6 年Sir, a very nice write-up on a very important and often overlooked seabed hazard.? I learned quite a few lessons on beachrock doing geophysical/geotechnical survey and burial feasibility on telecoms systems in the Persian Gulf.? The one that sticks with me most today is the importance of understanding how your survey system and sampling techniques relate to the actual geology...and how that translates into the results achieved by your burial tools.? Case in point:? CPT results shows a thin coarse unit within target burial depth along a route.? Geophysics barely indicated a faint horizon.? ?Vibracores retrieved broken gravelly calcarenite from the same depth with sandy muds above and below.? This seemed no issue at all for plow and jetting burial.? And indeed, the plow burial went perfectly fine through this unit.? But I got a call from the guys in the field that the diver jetting sled "hit rock that they COULD NOT bury through". I said that we had no rock at all in our geophysics or geotech results...were they sure?? ?I got back a video of jetting being stopped by beachrock, which at the time made no sense!? I knew there wasn't rock out there! After some more involved prodding and prying, it was found that the beachrock layer was less than 2 cm's thick, but not broken up. The large CPT could punch through it, yielding what looked like just a layer of coarse seds.? And the vibracore did the same and broke the calcarenite up into gravel-sized bits that looked like a gravelly layer when logged.? ?Burial by plow was able to get under this layer with the share and easily break it up.? But it sure did cause some headaches when it came to jetting!? So, just a long story to re-inforce your point on treating calcarenite/beachrock with an abundance of caution and the importance of understanding how your tools interact with the geology, not just looking at the apparent results they give.
Marine Technology, Geophysical & Hydrographic Specialist. Hydrospatial Advocate
6 年Great article Ian! Nice to see the old MGS bathymetry data of the submerged log-spiral embayment.