Geol 3000 Name Lab 11 Geologic Maps dominated by Igneous and Metamorphic Rocks Map MGS Miscellaneous Map M 68 Bedrock Geologic Map of Northeastern Itasca County Minnesota Jirsa 1990 This is a map of an Archean granite greenstone terrane in northern Minnesota In this terrane large scale faulting has segmented the crust into blocks here called areas While it is often possible to determine the stratigraphy within a given area it is often difficult to correlate the various lithostratigraphic or supracrustal sequences between areas Nevertheless regional correlations can be inferred based on similar stratigraphic sequences and regional up directions Based on the information presented on this map answer the following questions regarding the timing of events 1 What are Jirsa s interpretations of the relative ages of the six main lithostratigraphic sequences 1 oldest 6 youngest Bear Lake sequence Deer Lake sequence Joy Lake sequence Sherry Lake sequence Thistledew Lake sequence Wilson Lake sequence 2 Because most of the lithostratigraphic units on this map it is critical that we know which way is up so that we can tell when we are going up section younging or down section getting older A Sketch the structure symbol shows which way is up B Name two types of features might be present in these volcanic and sedimentary rocks that would offer evidence of up the direction 1 2 C What structural evidence do you think Jirsa used to make his interpretation about the relative ages for the segmented lithostratigraphic sequences 3 In the Wilson Lake Sequence there are over 10 intervals of iron formation that generally strike east west and are interlayered with several packages of volcanic rocks Awmv Awvs Awfv Which volcanic package hosts the oldest iron formation units Which volcanic package hosts the youngest iron formation units 4 The Deer Lake complex is an subvolcanic intrusive complex emplaced into the Deer Lake basalt forming the lower part of the Joy Lake sequence A Taking account of the structure of the Deer Lake basalts and the Joy Lake metasediments what type of intrusion shape best characterizes the Deer Lake Complex B The Deer Lake Complex is subdivided of two units pyroxene rich gabbro Adp and hornblende rich gabbro Adh Recognizing that the complex is faulted red lines and steeply oriented which unit is most typically above the other Top Bottom 5 Relative to the various rock units what is are the age s of the faults Older than unit Younger than unit Notice that there are two general directions of faulting a northeast trending set and a northwesttrending set What are the relative ages of these fault sets 6 Determine from the contact relationships with the surrounding country rocks whether the various granitic and gabbroic intrusions in the area were emplaced prior to during the tectonic event which deformed and metamorphosed the lithostratigraphic rocks i e are pre to syn kinematic or whether they were emplaced after the deformational event i e post kinematic Grantic Intrusions Acs Abm Aeg Agr Agg Awg Awg Gabbroic Intrusions Atmd Asmd Awmd Abmd Amd Ajmg Adh Adp 7 In the Bear Lake area of the map granite intrusions Awg and Agr are in contact with one another Which one is younger Why do you think so 8 The Kenora Kabetogama dike swarm Pd are 2 15 Ga mafic dikes that can be traced all over north central Minnesota These dikes probably represent the feeders to long since eroded flood basalts Dike swarms are usually intruded under tensional stresses sometimes caused by the impact of a mantle plume What was the principal orientation of those tensional stresses e g N S NE SW ENE WSW