Conglomerates and Breccias EarthCache

Conglomerates and Breccias are both rocks composed of coarse fragments of pre- existing rocks held together either by cement or by a finer-grained clastic matrix. Both contain significant amounts (at least 10 percent) of coarser-than-sand-size clasts, yet there are differences which stand out between a conglomerate rock and a breccia rock which we are going to look at for this earthcache.

Conglomerate rock (sometimes called Puddinstone)

• Is a clastic sedimentary rock that contains large rounded clasts/rocks.  The space between the clasts is filled with smaller particles and can include calcite or quartz cement that binds the rock.
• Clasts are normally larger than 2 mm, and can include pebbles, cobbles and small boulders
• The clastic material can be made up of any type of rock or mineral including quartzite, sandstone, limestone, granite, basalt, gneiss, as well as mineral particles such as quartz or feldspar.
• Clasts can come from sedimentary, metamorphic or igneous rock fragments     
• Often rounded and smooth from being washed downstream or in a current as a result of mechanical weathering.
• Conglomerate matrix binding the clasts together can be a mixture of sand, mud and chemical cement

Example of a Conglomerate or Puddinstone rock

Cementing Agents

• Cementation and Compaction are the two processes that bind the clasts together in one sedimentary layer. The first process is cementation which changes the sediment into rock by filling spaces around the larger sediments with smaller sediments and minerals. Common minerals that help with the cementation process are calcite and silica.

• Compaction aids in forming the rocks by squeezing together layers of sediment and reducing the thickness of the pore spaces around the clasts and sediments, which will lead to a tighter packed layer. The cementing agents found in breccia include silica, calcite (CaCO3), and iron oxides, and are the same as in conglomerate.

Breccia Rock

• Breccia is formed in a very similar fashion to conglomerate. The main difference between the two rocks is that breccia's rock fragments are very sharp and angular
• Also a clastic sedimentary rock composed of broken fragments of minerals held together by a fine grained matrix
• Breccias are consolidated rubble;
• The word has its origins in the Italian language, in which it means either "loose gravel" or "stone made by cemented gravel
• The cementing agents silica, calcite (CaCO3), and iron oxides are the same as in conglomerate.
• These rock fragments have not been transported by water, wind, or glaciers long enough to be rounded and smoothed like in the conglomerate.

  

  Types of Breccia

  Sedimentary – composed of 30% gravel size angular clasts produced by mechanical weathering or brittle deformation of nearby rocks.  Angular clasts in this breccia are chert fragments, and matrix is an iron stained mix of clay through sand size particles.

  Igneous – can form during the emplacement of igneous bodies by explosive exsolution of volatile phases and or explosive interaction of magma with ground water.   Igneous breccia can be further classified as either Volcanic or Intrusive

  Hydrothermal – are usually formed by hydrofracturing of rocks by highly pressured hydrothermal fluids and form at shallow crustal levels between 150 to 350 degrees Celsius when seismic or volcanic activity causes a void to open along a fault deep underground.

  Tectonic ( Fault breccia) - is a breccia that was formed by tectonic forces. In fault zones, where rocks or even continents slide past each other, breccia zones can be created that can vary from inches across to tens of metres across.

  Impact – sometimes called melt breccias formed by the fracturing and fusion of rocks under extreme pressures and temperatures rapidly induced during meteorite impacts.

   

  Famous conglomerates or Puddinstones

  Jasper puddinstone

  Hertfordshire puddingstone

  Roxbury conglomerate

   

  Logging Requirements

  Parking is available at Rotary Park.  During summer weekends it is very hard to find a parking spot.

  To Log this Earthcache

  Please send me your answers within 4 days of posting your found log. If there is more than one cacher in your party, include the names in your group. Only one person needs to send me the group answers. No spoiler photo's please. Found logs posted without proof you visited the site will be deleted.

  Questions

  Based on the description

  1.   What is the main difference between a conglomerate rock fragment and a breccia fragment?

  2.  How do the rock clasts inside a conglomerate achieve their smooth and rounded surface?

  3.  Clastic material can be made up of what type of rock or mineral?  Name two types?

  4.  Describe in your own words how the clasts are cemented together to form either a conglomerate or breccia rock.

  5.  The term breccia has its origins in what language?

   

  From the rock at the posted coordinates

  6.  Based on the description, you should now be able to determine that this is an example of a breccia rock.    Based on the types of breccia in the description, which one best describes this rock?

  7.  Look closely at the cementing agent.  Do you believe it is doing a good job or is water and chemical agents in the air causing this rock to erode, and if so, where is the main erosion taking place (top, bottom, sides).  Please describe?

  8.  You can see there is a crack along the top of the rock,  use a ruler to measure the average width of the crack?   Do you believe this rock is likely to continue to crack and split in two pieces or is this a minor crack?

  Optional:

  9.  Take a look on the surface of the rock and look for a fossil.  Take a photo and post it to your log of any fossil you observe.

   

  Please do not post any photos that show the rock with any spoiler images.  Photos showing spoilers will be deleted.

  References:

  https://geology.com/rocks/breccia.shtml

  https://geology.com/rocks/conglomerate.shtml

  

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