Changing Climate and Wind Patterns Revealed in Indiana's Fair Oaks Dunes

Fair Oak Dunes (FOD) cover over 1100 square miles in north-central Indiana. Careful study of dune morphology reveals three types of dunes in regards to their size. The first order forms are compound parabolic dunes that reach over five miles in length and have the apex of parabola pointing in a southwesterly direction. The spacing between these dunes is three to five miles. The second order dune ridges are compound parabolic dunes that range in size from one to three miles in length with spacing of about one mile between the ridges. Both, the second order and the third order dunes have the apex of parabola pointing in northeasterly direction, opposite of the first order dunes. The third order dune ridges are simple parabolic dunes that reach up to half mile in length and are 25 to 30 feet tall in western part to over 45 feet in the eastern part of the FOD. All dunes are fixed by lush vegetation. Preliminary grain size analyses indicate that north part of FOD has coarser sand (0.283 mm) than southern part (0.197 mm), while eastern part (0.271 mm) is coarser than the western part (0.223 mm). This grain size distribution is in accordance with initial interpretation of dune morphology. Strong northeasterly winds associated with anticyclone were prevalent in early dune formation about 14,000 years ago near the end of last glacial. The finest particles were blown south and southwest from the source area which was north and east of the present dunes. Cyclonic southwesterly winds become dominant in Holocene and caused a reworking of the original large dunes into smaller forms as well as removal of some of the finest particles back to the original source to the northeast. Limited vertical dune profiles indicate that below the 5 feet of bioturbated surface layer are alternating light layers (3 to 5 inch thick) and dark laminae (1-2 inches thick). Dark laminae consist of quartz grains with `hairy' surfaces covered with reddish iron oxides or clays. They contain twice as much silt fraction and are more indurated and resistant to weathering than light layers separating them. Are these alternating layers depositional or postdepositional structures? Depositional origin would indicate episodes of drier climate and strong winds creating light, coarser layers while dark laminae would be indication of wet climate and weak winds transporting only dust particles. Postdepositional origin would indicate formation of "dissipation structures" created by soil water translocation of fines during humid climate. Many of the questions about timing of original dunes and their later transformation as well as timing of dark laminae and light layers within the dunes will hopefully be answered by OSL dating.