GY 111 Lecture Notes D Haywick 2008 09 1 GY 111 Lecture Notes Sedimentary Environments 3 Beaches Lecture Goals A Types of modern beaches B Wave and beach dynamics C Beach sedimentation Reference Press et al 2004 Chapter 7 17 Grotzinger et al 2007 Chapter 20 p 485 491 GY 111 Lab manual Chapter 3 A Types of modern beaches I ve lived in Mobile for about 17 years During that time I ve complained about the weather too hot too wet too many damn hurricanes Mobile drivers Airport Blvd the weather again and our crappy local news media The one thing I have never complained about are the local sedimentary environments In particular our beaches are the envy of the world really geologists come here to study beach dynamics It is true that some of our beaches are in trouble more about this shortly but we can still learn an awful lot a beach sedimentation in our area First of all it is important to note that there are lots of types of beaches Most are sandy in that sand is deposited on them but some are sites of gravel deposition These gravel beaches are usually called shingle beaches and they are common along rocky coastlines and or coastlines that are in cooler temperate areas e g New Zealand Canada The reason is that colder areas have less intense chemical weathering resulting in larger sedimentary particles Beach gravel tends to be very well rounded and discoidal in shape which makes excellent skipping stones GY 111 Lecture Notes D Haywick 2008 09 2 Sand beaches are more typical for our part of the world They can be attached to the mainland mainland attached beaches see picture on the previous page of a beach in New South Wales Australia or detached barrier islands see picture to left on this page The side of the beach that is in direct contact with ocean waves is called the open side because it is open to the full force of the wind and waves On barrier islands the side of the beach that in on the lagoonal side is called the sheltered beach because it is sheltered from the most powerful waves Most people regard the beach as the part of the shoreline where you sit down to get a tan Geologists actually have a more inclusive definition of the beach It is the sedimentary environment that extends from the sand dunes landward of the high tide line to the point offshore where water depth reaches 10 m Seaward from here is the continental shelf The actual width of the beach depositional environment is highly variable In southern Alabama the beach extends for about 8 10 km south of the shoreline because our coastline is so flat I ve complained about this several times in my 17 years of residence in Mobile too In areas where the coastline and shelf are less flat usually a tectonically active area like western North America the beach is far less wide In some places it may be only a couple hundred feet wide Another thing that is highly variable on beaches is the size of waves that impact on them Our beaches as nice as they are sand wise totally suck as far as waves are concerned With the exception of hurricane induced waves normal fairweather waves seldom exceed a foot or two in height At some beaches e g western Australia eastern South Africa Hawaii fair weather waves can reach 3 7 m I suppose this is why the world s best surfers come from Hawaii Australia and South Africa Waves are the major form of energy controlling sedimentation on beaches and this is why we need to spend more time discussing them B Wave and beach dynamics Ocean waves are formed by the wind and wind speeds as low as 5 kph 3 miles per hour are sufficient to get them going The faster the wind the bigger the waves get Wave size called the wave height is normally measured from the base of the trough to the top of GY 111 Lecture Notes D Haywick 2008 09 3 the crest Since waves are produced by the wind they are largely a surface phenomenon If you sink below the surface e g you are SCUBA diving you will feel the wave motion diminish the deeper you go Now about the wave motion It is not as many people think simply up down It is more orbital or orbicular than simply up and down As a wave approaches a floating object the object is drawn toward the crest at the same time as it is being lifted upward When the crest passes the floating object falls at the same time as it slides forward After one wave has completely passed the floating object has traced a circular pathway see next cartoon The size of these oribitals decreases with depth By the way the wave base can be determined through a very simple mathematical relationship Wave base wavelength 2 L 2 As waves approach the shoreline a point is reached where the seafloor is less deep than the wave base At this point bottom sediment is agitated by waves The waves also start to change their character The wavelength starts to decrease and the wave length starts to increase The waves are said to be building and this interval is called the buildup zone When the waves have grown too high they spill over as breakers The largest breakers e g near Hawaii develop classic rip curl forms that can be surfed through The collapsed breakers push a sheet of water onto the beach swash or surf zone see picture to right that is the most energetic part of the whole environment The constant washing back and forth in the GY 111 Lecture Notes D Haywick 2008 09 4 swash zone causes all of the sand grains to be extremely well rounded and well sorted The dominant mineral on the beach at least in our part of the world is quartz Should this sediment be lithified it would be converted into a quartz arenite sandstone C Beach sedimentation Okay so I already mentioned that the most common type of sedimentary deposit on a beach is quartz and that quartz arenite sandstones are common beach deposited sedimentary rocks But not all quartz sand is the same The beach depositional environment consists of a lot of smaller components The proper term for this is morphology and the ideal beach morphology is shown at the top of the next page Healthy beaches are those where the forces of construction e g sedimentation are greater than or equal to the forces of erosion Beaches are said to be prograding if they are advancing toward the sea Sadly the majority of our beaches are experiencing more erosion than sedimentation They are said to be retreating Healthy beaches have several morphological features as illustrated in the diagram above Please note this diagram is highly exaggerated in the vertical dimension by about 5 times The highest