Large-Scale Channel Development Within Capitan Reef Facies--Evidence from Carlsbad Caverns

Gill Harwood

Facies mapping within Left Hand Tunnel of Carlsbad Caverns has revealed a channel, some 230 m wide by over 25 m deep, which developed within the Capitan reef and is surrounded to both the east and west by reef facies sediments. This channel contains greater than 10-m scale reworked angular reef blocks. Many of these derived blocks contain cement botryoids that have been truncated at block margins. These cements, therefore, formed before redeposition of the blocks and evidenced rapid cementation within the reef facies. This cementation enabled the initiation of jointing within the reef with subsequent spalling of large blocks into the developing channel. The orientations of these blocks, determined from geopetally floored intrareef cavities, are random and many are overtur ed. In some of these cavities, a later postblock reworking phase of internal sediment is horizontal. The blocks are larger toward the western margin of the channel and are interbedded with finer (centimeter to meter-scale) rounded to angular reef rubble. Winnowed reef sediments, commonly brachiopod rich, are more abundant toward the eastern channel margin.

The channel formed during reef development as the overlying sediments, initially wackestones/packstones but later reef framestones/boundstones, prograded over the channel talus. Channel initiation may have resulted from (1) foundering of the reef front, with subsequent shoreward propagation along a northerly lineament, or (2) synsedimentary tectonic movements. The channel remained open for some period; the large blocks are repeatedly interbedded with skeletal packstones, deposited into the channel by westerly directed currents, and breccia/rubble beds.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.