The Antiquity of Castleguard Cave, Columbia Icefields, Alberta, Canada∗

Several sections of Castleguard Cave contain abundant, massive fossil speleothem deposits. Twenty-one samples have been dated by the 230Th/234U method. Ten of them are older than 350 ka (1 ka = 1000 yr before present). Included in this group are (1) a massive in situ flowstone in the Waterfall Chamber along First Fissure, (2) a prominent indurated stalagmite projecting through the false floor in the Grottoes, and (3) the base of a large flowstone boss near the start of Second Fissure. More recent deposits include a false floor overlying laminated clay in the Grottoes (144 ka), a flowstone veneer apparently overlying laminated silt in First Fissure (100 ka), and a similar veneer between Holes-in-the-Floor and Second Fissure (38 ka). Speleothems (1) and (2) above have been found to be magnetically reversed, indicating an age of greater than 720 ka, but probably less than 1 ma from examination of their 234U/238U ratios. These ages clearly demonstrate the antiquity of the cave and show that (1) First Fissure, the Grottoes, and Second Fissure were vadose by 720 ka, (2) the silt deposits along First Fissure and in the Grottoes are older than 100 ka, and if of glacial origin, must therefore be from the penultimate glaciation, or earlier, and (3) the presently active (but unexplored) drainage system of the cave is likely to be very old and, therefore, well developed.