Geological and mineralogical interpretation of shallow sediments from the northeastern Grand Banks of Newfoundland
Authors: Segall M P , Lewis C F M , Barrie J
V , Maher M L J 
Published: December 2011

The clay-sized minerals from 13 sites (51 samples) on the northeastern Grand Banks of Newfoundland have been analysed using X-ray diffraction and scanning electron microscope techniques. This data has been incorporated with pebble lithological and borehole
stratigraphic analysis to provide additional information regarding the physical and chemical processes in the region from approximately mid-Tertiary times to the present. Two chemical environments are interpreted for the shallow geology of the northeastern Grand Banks of Newfoundland based on clay
mineralogy, color and the presence/absence of shells. In addition, three major stratigraphic units have been further defined to depths of 130 m below the seabed using the preceding criteria. These are as follows: 1. the uppermost Late Pleistocene/Holocene sediments; 2. the uppermost weathered zone
of Tertiary marine deposits; 3. the underlying thick sequence of Tertiary marine deposits. 1. The uppermost Late Pleistocene/Holocene sediments which consist of gravels and sands C&amp;lt; 4 m thick; locally as only a thin veneer) are predominantly clay poor. Although the sands and gravels
probably consist of reworked material, the clays generally reflect a northern provenance, having high average concentrations of illite (28%), quartz Cl4%), and feldspar (19%) along with a chamosite component. These clays have predominantly been deposited as a result of ice transport
mechanisms. Where the signature varies to include expandable minerals (0-24%) and kaolinite (0-15%), it is probable that clays found within the uppermost zone of weathered Tertiary marine sediments (directly underlying the Late Pleistocene/Holocene sediments) have been incorporated into the
Late Pleistocene/Holocene regime by scouring ice which has broken through the desiccated crust of the unconformity. They could also be remnants of the underlying Tertiary sediments that were not completely removed from the area during transgression approximately 10,000 years ago. An increase in
sodic feldspar contents as well as a general increase in the angularity of clay-sized minerals is seen in deeper waters (125 m) to the east of the Hibernia P-15 well site. This could reflect the less energetic reworking of particles at these depths under the present oceanographic regime. 2. The
uppermost zone of weathered Tertiary marine sediments exhibits relatively high contents of kaolinite (7-18%) and high concentrations of expandable minerals (avg. 29%), as well as a clinochlore component not seen in Late Pleistocene/Holocene clays. The sediments change from a pale yellow
color in the Late Pleistocene/Holocene to a dull olive brown in the underlying Tertiary unit. It is expected that this relatively thin subsurface weathered zone is composed of sediments derived by reworking and chemical alteration of the directly underlying Tertiary marine sediments. The presence of
kaolinite is attributed to the leaching environment which accompanied subaerial exposure of the study area approximately 15,000 to 10,000 years ago. For the formation of kaolinite from such minerals as montmorillonite, labradorite and illite, the acidity would have had to have increased to a pH of 4
and conditions of oxidation would have prevailed. The absence of shells from this interval is a further indicator of an acidic environment. This weathered Tertiary unit has an increase in silt/clay constituents and is hard and dense. The latter factor is attributed to subaerial weathering processes.
3. The lowermost unit (sampled to 130 m below the seabed) is comprised of eastward thickening and dipping olive colored Tertiary marine sediments which were deposited during transgressive and regressive phases. These sediments show a general coarsening upward trend, although all contain a
predominance of very fine sand/silt/clay components. Expandable minerals (avg. 45%) constitute the highest abundance of claysized materials, which vary from almost 100% montmorillonite to 100% swelling chlorite. No pebbles &amp;gt; 28 mm were found in any of the cores, with the
highest percentage of pebbles existing in samples from the Ben Nevis I-45 borehole.