Miocene Lithofacies in Southeast Louisiana

ABSTRACT The gross lithofacies of the Miocene are defined in terms of sand per cent and, on the basis of the vertical distribution of sand per cent, southeast Louisiana is divided into four lithofacies provinces. Because stratigraphy and structure are intimately related in the Miocene of this area, the lithofacies are interpreted in terms of the structural history of the various provinces. PROVINCE I is stratigraphically and structurally the least complex of all the provinces and is characterized by a simple regressive facies. This province includes most of the lower Miocene trend of southeast Louisiana north of Terrebonne and Lafourche Parishes and north of an east-west line approximately through the middle of Jefferson and Plaquemines Parishes. Excluded from Province I is that area in northern Jefferson and Plaquemines Parishes which encompasses the "Harang Embayment". This embayment is considered a portion of Province II. PROVINCE II includes the northern portions of Terrebonne and Lafourche Parishes and southern Jefferson and Plaquemines Parishes. The lithofacies of this province reflect an overall regressive sequence with a major regressive-transgressive cycle developed at depth. This cycle is related to the pulses of movement of the major growth faults which characterize Province II. The age of this regressive-transgressive cycle becomes progressively younger in a basinward direction within Province II as does the age of the major growth faulting. The "Harang Embayment" is included in Province II and is characterized by a major regressive-transgressive cycle developed at depth. However, only one such cycle is developed in the "Harang Embayment" and not a progression of successively younger cycles. The regressive part of this cycle is represented by the widespread Cristellaria "I" -- Cibicides opima sands and the transgressive phase is represented by the overlying Planulina harangensis shale. PROVINCE III encompasses the southern portions of Terrebonne and Lafourche Parishes and coincides with the "Terrebonne Trough". This province is characterized by an extensive development of the massive sand facies which reflects the effect of a young period of major growth fault movement. Because the faults are young and had the major portion of their growth during the deposition of the younger sand rich sediments, the expanded downthrown section resulting from this growth consists of sediments of high sand per cent. PROVINCE IV includes the lower portion of the Mississippi delta and portions of the adjacent offshore area. The lithofacies reflect numerous regressive-transgressive cycles and a poor development of the massive sand facies. Structurally this was the most unstable of the provinces, and the stratigraphy is the most complex.