Abstract:
The aims of this work were to test and refine the established models of preservation and distribution of sedimentary organic matter. Two test areas with a well-documented geological framework were selected in the Miocene of the USA, each one representing a completely different depositional environment: the New Jersey continental margin is characterized by an oxic environment with high sedimentation rates, whereas the Monterey Formation in California is marked by a dysoxic environment with low sedimentation rates. The Miocene of four ODP sites was sampled in the New Jersey continental margin, which represents a classical example of siliciclastic passive margin. These sites provide a transect from shelf to slope and are further correlated with gamma-ray and seismic data. The latter provide a pre-established sequence stratigraphic framework, within which the distribution of organic matter can be tested. In this case, the Continental/Marine ratio based on palynomorphs turns out to be the most reliable palynofacies parameter. It shows an overall decrease from shelf to slope. On the slope, the progradation of clinoforms is recorded by successive increases in this ratio. By contrast, it displays a decrease in transgressive intervals. The overall trend of this ratio can be correlated with the global sea-level curve, thereby confirming that it can be used as a proxy for palaeoenvironmental changes. The gamma-ray is positively correlated with the Continental/Marine ratio in a proximal setting, whereas it reacts in the opposite way in the distal part of clinoforms, where all parameters display a deepening-up trend. Subsequently, the gammaray can be used as an indicator of transgressive/regressive trends. The correlation between palynofacies trends and seismic data has permitted the refinement of a sequence stratigraphic interpretation. The main seismic markers are often coincident with condensed intervals. Furthermore, in the New Jersey margin, the positive correlation between gamma-ray, total organic carbon (TOC) and Continental/Marine ratio confirms that the hydrodynamic behaviour of continental sedimentary organic matter is close to that of clay/silt size particles. Amorphous organic matter (AOM) is also a significant constituent of the palynofacies. It has been subdivided into three types: dark-brown, light-brown and fine-AOM. In this case, the origin of AOM is continental. On the shelf, the distribution of AOM is a direct function of the terrigenous supply, whereas, in the distal part of the clinoforms, it is dominated by palaeoenvironmental conditions associated with a strong bacterial activity in these less oxic waters. The Miocene Monterey Formation was deposited in a small-size extensional basin associated with a dynamic prograding margin. Organic matter was studied in the El Capitan field section. It is mainly composed of amorphous organic matter, which has been subdivided into the same three types as in the New Jersey margin. Dark-brown AOM is more abundant in marls enriched in phosphates of the pristine type, whereas light-brown AOM predominates in marls with none to rare phosphates. The strong association of dark-brown AOM with phosphates seems to indicate that this type of AOM in the Monterey Formation corresponds to the degradation and accumulation of organic matter in bacterial mats, in a dysoxic environment and with very low sedimentation rates. Light-brown AOM is associated with sediments deposited and/or reworked in slightly more oxic bottom-water environments. Consequently, in this case, AOM has proven to be a useful palynofacies indicator for determining palaeonvironmental conditions at the water-sediment interface. Finally, the comparison of both case studies has highlighted the significance of AOM in palynofacies studies. In SEM and blue-light fluorescence, dark-brown and light-brown AOM are very similar at both locations. However, dark-brown AOM in the New Jersey margin is thinner than dark-brown AOM in the Monterey Formation and TOC's are much higher in the latter case. It appears that, although similar in appearance, the material forming dark-brown AOM has a different origin: its source is mainly continental in the prograding margin of New Jersey and mainly marine in the Monterey Formation. In both cases, bacterial activity plays the major role in the degradation and concentration of AOM.