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U of M GEOL 3000 - GEOL 3000 Laboratory

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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 northwest-trending 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, …)


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U of M GEOL 3000 - GEOL 3000 Laboratory

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