The Twin Knobs #1 diatreme (TK1), delineated in 1981, is located 3.8 km southeast of Murfreesboro, Pike County, Arkansas and is within the northeast-trending group of pipes, plugs, and dikes that comprise the Murfreesboro district. TK1 is an elongate, bilobate pipe, oriented north-south, and approximately 0.05 km 2 in surface area. The diatreme penetrated the Lower Cretaceous Trinity Formation, whereas the Upper Cretaceous Tokio Formation overlies portions of the pipe. These relationships suggest an emplacement age of early Late Cretaceous, which is contemporaneous with isotopic ages obtained at the Prairie Creek lamproite complex (97 to 106 Ma; Gogineni and others, 1978). Rock types at TK1 include magmatic lamproites, sandy tuffs and breccias, and epiclastics. The magmatic breccias are porphyritic, with euhedral olivine, plus pyroxene and phlogopite phenocrysts set in a cryptocrystalline or glassy groundmass that also contains priderite, perovskite, and chromite. The sandy tuffs and epiclastics contain up to 30 percent quartz grains, in addition to lamproite and sedimentary rock fragments, set in a matrix of calcite, sericite, or fine-grained to cryptocrystalline glass. The average diamond grade from bulk sampling of the phases is 0.17 carats per 100 metric tons. Diamond crystal habits include octahedra, tetrahexahedra, macles, and fragments. Megacrysts of picroilmenite, chrome spinel, clinopyroxene, orthopyroxene, garnet, and nodules of eclogite, peridotite, Iherzolite, websterite, as well as crustal xenoliths, were recovered during sampling. Whole-rock and trace element geochemistry strongly suggests that Twin Knobs #1 has more affinities to olivine lamproite than to kimberlite.

Abstract: The Glen Rose Formation of the Comanchean Series represents the second circum-Gulf carbonate shelf that extended fromFlorida to Mexico. The Glen Rose comprises limestone, dolomite, and thin interbeds of marl and calcareous shale that overlie the HenselSandstone and underlie the Fredericksburg Group in Texas. The Glen Rose is here formally divided into Lower and Upper, mappablemembers separated at the top of the regionally persistent “Corbula” Marker, and a boundary stratotype section is designated. The age of the Glen Rose Formation ranges from latest Aptian to near the end of the Early Albian, from approximately 113.3 Ma to 108.0Ma, and encompasses four ammonite zones. Three local assemblage zones facilitate correlation of Glen Rose outcrops in Texas. The Saleniatexana Credner Assemblage Zone spans a marl, 3 to 4 m thick, with a diverse shelf biota in the upper part of the Lower Member. The “Corbula” Range Zone is at the top of the Lower Member. The Loriola rosana Cooke Assemblage Zone is in the middle part of the UpperMember. The Glen Rose together with the underlying Hensel Sandstone comprise at least five medium-scale depositional cycles separatedby transgressive disconformities. Two types of biotic accumulations are well developed in the Lower Glen Rose Member, coral-rudist assemblages and caprinid-dominated assemblages. Coral-rudist biostromes crop out at the Narrows of the Blanco River and had no bathymetric relief and do notqualify as “reefs”. Colonial corals are common and are part of a Tethyan fauna; toucasiids and other mollusks comprise a diverseassemblage. Coral diversity is underestimated because of incomplete preservation and sampling. These biostromes are overlain bygrainstone capped by a subaerial contact that serves as a sequence boundary between cycles one and two. The younger caprinid biohermsat Pipe Creek have up to 10 m of depositional relief. The bioherm facies grade landward into shoreface grainstone and seaward into shelfwackestone. The caprinid species, Coalcomana ramosa , dominates this low-diversity assemblage and is endemic to the Caribbean Province.The bioherm facies are overlain by dolomitic, stromatolitic facies with dinosaur tracks. The contact with the overlying Salenia Zone is asequence boundary between cycles two and three. Two more long-term cycles may be identified in the Upper Glen Rose Member. The coral-rudist biostromes and the caprinid bioherms formed paleocommunities on the landward ramp of the interior marine shelf.The coral-rudist biostromes developed below normal wave base but above storm wave base and shoaled above wave base and weresubaerially exposed. The caprinid bioherms formed on a ramp and grew into the zone of normal wave action in mainly normal marinesalinities.