LAKE SHORE TRAPS
The Lake Shore Traps
are flood basalts interbedded with coarse-grained siliciclastics of the Copper
Harbor Conglomerate. “Traps” is derived from a Swedish word, and the term
is used in geology to refer to rock outcrops that weather into a large-scale
stair-step pattern. Multiply stacked lava flows generally weather this
way. In western India, a famous Cretaceous-Tertiary boundary-aged flood
basalt is called the Deccan Traps.
Flood basalts that filled an
ancient continental rift valley outcrop throughout Michigan's Keweenaw
Peninsula. Most of these lava flows are found in the 1.093-1.097 billion
year Portage Lake Volcanic Series. After the main episode of flood basalt
eruptions ceased, conglomerates and sandstones of the 1.085-1.093 billion year
Copper Harbor Conglomerate were deposited atop the lava flow succession.
The rocks of the Lake Shore Traps Member (1.087 b.y.) represent a short-term
resumption of flood basalt eruptions.
The four photos below show
outcrops of Lake Shore Traps flood basalts at Eagle Harbor Lighthouse, on the
western end of Eagle Harbor's entrance (see
map).
Lake Shore Traps Member flood basalts (upper Copper
Harbor Conglomerate, Oronto Group, upper Mesoproterozoic, 1.087 b.y.) at Eagle
Harbor Lighthouse, UP of Michigan, USA.
Lake Shore Traps Member flood basalts (upper Copper
Harbor Conglomerate, Oronto Group, upper Mesoproterozoic, 1.087 b.y.) at Eagle
Harbor Lighthouse, UP of Michigan, USA.
Lake Shore Traps Member flood basalts (upper Copper
Harbor Conglomerate, Oronto Group, upper Mesoproterozoic, 1.087 b.y.) at Eagle
Harbor Lighthouse, UP of Michigan, USA.
Lake Shore Traps Member flood basalts (upper Copper
Harbor Conglomerate, Oronto Group, upper Mesoproterozoic, 1.087 b.y.) at Eagle
Harbor Lighthouse, UP of Michigan, USA.
Another good place to
examine rocks of the Lake Shore Traps is the Porcupine Mountains of northern
Michigan. The mountain scenery is probably best appreciated from an
overlook at Lake of the Clouds. The overlook itself is a knob of Lake
Shore Traps flood basalts.
Lake of the Clouds, Porcupine Mountains, UP of
Michigan, USA.
Lake Shore Traps Member flood basalts (1.087 Ga) at
Lake of the Clouds overlook, Porcupine Mountains, UP of Michigan, USA.
The lava beds here have nice glacial grooves and striations.
Lake Shore Traps Member flood basalts (1.087 Ga) at
Lake of the Clouds overlook, Porcupine Mountains, UP of Michigan, USA.
The outcrop is glacially smoothed & striated.
Lake Shore Traps Member flood basalts (1.087 Ga) at
Lake of the Clouds overlook, Porcupine Mountains, UP of Michigan, USA.
The outcrop is glacially smoothed & grooved.