2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 3
Presentation Time: 2:00 PM


MILLER, Thomas E., Department of Geology, Univ of Puerto Rico @ Mayagüez, POB 9017, Mayagüez, PR 00681, bzekarst@hotmail.com

Flow direction in small rivers of the north central Puerto Rican karst is influenced dominantly by dip, strike, fault systems, basement topography, and discharge volume. After integrating on non-carbonate rocks, the Río Camuy sank underground in large caves mapped 3½ km on-strike, to merge eastward with the surface course of the Río Tanamá. Both followed a fault system 6 km NE as a single river before turning north on dip. After 100m of vertical incision a resistant basalt was exhumed prior to 80 Ka; the tributary Camuy abandoned the strike segment and swiveled north to a cave gradient directly down-dip to the Atlantic.

Data from other valleys in this karst suggest that a mean annual discharge of about 60-80 cfs is the threshold to sustain surface flow in the humid tropics, where areal flows are 2-3 cfs mile-2. The loss of the Camuy (~50 cfs) reduced the Tanamá (now 48 cfs) below the limit of surface flow, and it was subsequently diverted underground. Abandoned stream terrace straths, and 2-3 levels in seven caves through which the Tanamá now flows, largely correspond to knickpoints in its present profile. The Río Camuy cave system records a vertical series of knickpoints in its six subsurface conduit levels.

Sapping and collapse of the Oligocene Lares Limestone overlying weak clastics of the San Sebastián Formation promotes a northward-migrating karst, with growth of east-west strike valleys at its southern boundary. Larger through-flowing rivers will complete holokarst development as they incrementally behead and further reduce catchments of the smaller streams.