Sedimentary dikes in the Permian (Guadalupian) Capitan Reef of West Texas and New Mexico are composed of siliceous sand, skeletal carbonate, or cement. Skeletal-carbonate neptunian dikes appear to be a relatively uncommon type; only four are known by the authors, but they are important in providing information about platform growth, sediment sources, and dispersal on the platform, and the biota that were present on the platform.

The dikes occur within massive poorly sorted skeletal grainstones typical of the reefal part of the Capitan Formation (the Capitan-massive) that is correlative with upper Guadalupian Yates B strata on the platform. They formed during later Yates B, Yates C, and Tansill deposition on the platform, following closely a major period of progradation of the platform margin. The dikes are approximately vertical, and parallel to and within about 100 m of the platform margin. Fissure formation was a probable consequence of flexing and fracturing of lithified shallow-water Capitan-massive over compacting unconsolidated slope and basinal sediment.

At least three stages of opening and filling are recorded in the largest and most complex dike, with each sequential fissure opening forming on the basinward side of the previous cemented filling. In two of the dikes, centimeter-thick deposits of vertically laminated peloidal grainstone, which formed in situ within microbial films on the fissure wall, preceded filling by skeletal sediment.

Skeletal fossils within two of the dikes are well preserved and represent organisms that either were living within the fissures or were living on the adjacent sea floor and were washed into the fissures soon after death. Other skeletal grains in these dikes, and all of those in the other two dikes, are abraded, strongly micritized, and encrusted with microbial micrite, indicative of having lain on the sea floor for a considerable time before being deposited in the fissures. Fossils in the dikes are characteristic of the reefal and adjacent outer-shelf facies tracts, but they are a biased record of the biota of that depositional area in that they consist largely of vagile or only weakly attached members of the dweller and baffler guilds living on the reef surface and of organisms living within the fissures; members of the reefal framework communities are largely absent.

The absence in the dikes of fossils diagnostic of more distant facies tracts indicates that there was little across-platform movement of carbonate sediment on the platform. Deposition of terrigenous sediment is correlated with a sea-level low-stand, indicating that land-derived sediment moved across the platform otherwise infrequently.

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