AN INTEGRATED STUDY OF DIAGENESIS AND SEQUENCE STRATIGRAPHY IN SHOREFACE SANDSTONES: PONTA GROSSA FORMATION (DEVONIAN), PARANA BASIN, BRAZIL
Early, near-surface diagenetic alterations include the formation of grain coating and ooidal berthierine, ooidal Fe-oxyhyroxides, grain-coating micro-quartz, pyrite and siderite in transgressive system tracts (TST), particularly below the transgressive (TS) and maximum flooding surface (MFS). Grain-coating micro-quartz and berthierine, which has been subjected to partial chloritization, retarded cementation by quartz overgrowths, and hence preservation of reservoir quality. The formation of siderite, pyrite, micro-quartz and berthierine was presumably aided by low sediment supply and elevated organic matter content, which enhanced the microbial suboxic to sulfate-reduction diagenesis. Low rates of sediment supply also enhanced the formation of ooidal Fe-oxyhydroxides.
The sand bodies which have grain replacement by kaolin, burial transformed into dickite, were interpreted to be near a sequence boundary in lowstand system tract (LST) due to meteoric water incursion during a relative sea-level fall. The same feature was also identified in the late highstand system tract (HST). Albitization of plagioclase was encountered in (TST) and (HST) sandstones. The deep burial calcite cementation in such sandstones occurs with poikilotopic texture. The fine-grained sandstone with berthierine coatings and ooids, and iron oxide ooids were related to sequence boundary and subsequent transgressive (TS), because the conditions required to the precipitation of these minerals, including slow net sediment accumulation, mainly occurring during the initial marine transgression.
This study demonstrates that the spatial and temporal distribution of diagenetic alterations is better elucidated when linked to the sequence stratigraphic framework of the shoreface sandstones.