GEOL 3000 Name Lab Exercise 2B Lumping and Splitting The Art of Creating Map Units One of the most interpretive aspects of creating geologic maps is defining map units A map unit is a collection of rock types that share a common age and mode of origin In some cases deciding what rock types should be grouped together is straight forward as there is one main type with well defined boundaries with other rock types e g fine grained gabbro within a mafic dike In many cases however there are gradations between rocks of a particular class e g limey shale shale silty shale and gradational contacts with surrounding rocks Then it becomes more challenging to define the rock types that will comprise a particular map unit One must decide if the difference are significant enough and the areal distribution of the rock types is significant enough at the scale of the map to warrant breaking out the units splitting into separate subunits Or should the various rock types be consolidated into one unit lumping There are no set rules for how this iterative process of lumping and splitting should be done which is why the name on a map is important Each mapper must decide what descriptive criteria they will use to define their the map units For this exercise we will be working with the Geologic Map of Minnesota Morey and Meints 2000 to evaluate how these authors chose to group rock types into map units Several copies of the map are laid out in lab but it can also be accessed as a pdf file from the class website 1 For each of the major geological eras identify a unit that contains a large variety of rock types lumped and one that has only a few main rock type split Fill out the table below with the unit and its map symbol the rock types it contains and the general area of the state where the unit occurs note that not all units are contiguous and may occur in multiple areas 8 pts Paleozoic Era A Lumped Unit Map Symbol Rock Types Location B Split Unit Map Symbol Rock Types Location Mesoproterozoic Era A Lumped Unit Map Symbol Rock Types Location B Split Unit Map Symbol Rock Types Location Paleoproterozoic Era A Lumped Unit Map Symbol Rock Types Location B Split Unit Map Symbol Rock Types Location Late Archean Era A Lumped Unit Map Symbol Rock Types Location B Split Unit Map Symbol Rock Types Location 2 As we will discuss in Week 5 s lecture there is a hierarchy to the classification of map units For stratified rocks sedimentary and volcanic the groupings are Supergroup Group Formation Member Most commonly units with formation or member status are shown discrete units on geologic maps Look through the description of map units and find two examples of the progression from Group to Formation be sure to list all the formational subdivisions of the group 2 pts Group Formations Group Formations 3 Devonian aged rocks which occur in southeastern Minnesota are distinguished into groups formations and members Draw a flow chart showing the subdivisions of the Devonian strata as described in the Description of Map Units Put a next to those subdivisions that are shown as their own map units 2 pts Groups Formations Members 4 Note that several of the Devonian units are broken out split in Mower County but all are lumped into a Devonian Rocks undivided unit in Freeborn County Why do you think the mappers did this Clue check out the Density of Bedrock Outcrop diagram 1 pt