ABSTRACT

Numerous igneous dikes intruded Jurassic strata around the Monroe Uplift in northeastern Louisiana during Late Cretaceous time. One of these dikes was encountered in a conventional core through organic-rich lime mudstones and siltstones of the Smackover Formation at a depth of 2.1 km in the Carter #1 Hope-Fee well, Morehouse Parish. The intruded interval consists of organic-rich siltstone layers alternating with organic-poor clean lime mudstone layers. Each layer type is internally laminated and ranges in thickness from 2 to 50 cm. The dike is ten meters thick and exhibits holocrystalline but porphyritic texture. It is composed of complexly zoned phenocrysts and microphenocrysts of titanaugite, analcime, sphene, and apatite. Microprobe analyses of titanaugite and bulk rock trace element compositions are indicative of highly undersaturated, moderately magnesian igneous rock, similar to other Late Cretaceous igneous rocks in the region. Sodium concentration in the dike decreases whereas K, Ba, Rb, Sr, and Cs contents increase toward the dike margin. These trends were caused by expulsion of high temperature fluids from a volatile-rich melt and by exchange with the host rock.

The intrusion produced asymmetric contact metamorphic effects to six meters above and four meters below the dike margins. Metasomatic changes are observed in textures, mineral assemblages, mineral chemistry, and calcite stable isotope (¹⁸O, ¹³C) compositions of the contact aureoles. Within one meter of the dike, contact metamorphic assemblages include calcite + K-feldspar + albite + diopside + grossular + apophyllite, + pectolite, and pyrite. Farther from the dike, poikiloblastic calcite locally includes sprays of pectolite + pyrite in a fine grained matrix of calcite + quartz + K-feldspar + albite + diopside. Calcite ¹⁸O and ¹³C compositions are -15 PDB and -7 PDB, respectively, at the dike margins and increase exponentially to -4 PDB and +4 PDB, respectively, in unmetamorphosed carbonates. Calcite ¹⁸O values in the contact aureoles were influenced by temperature and by ¹⁸O compositions of the host rock, formation waters, and magmatic fluids. The ¹⁸O geothermometer suggests a temperature range of 340 to 440°C at the dike margin. Calcite ¹³C signatures in the contact aureoles were controlled by temperature-dependent exchange between organic carbon and carbonate carbon.

Heating from the Monroe igneous intrusions caused over maturation of organic matter and generation of large volumes of hydrocarbon gases which migrated into younger reservoirs. Locally, high geothermal gradients caused by igneous intrusions may have promoted hydrothermal circulation and upward transfer of solutes during late Mesozoic and early Cenozoic.