Six different lithofacies are recognized. The lithofacies are coded using texture (W = wackestone and P = packstone) and a number that increases as the lithology becomes less abundant. The lithofacies reflect the different texture, grain composition, grain size, and sorting as seen in thin sections. All lithologies are unlithified, and the main diagenetic features are a few dolomite rhombs and small calcite and pyrite crystals inside foraminifer tests. Figures F2 and F3 show the stratigraphic section and photographs of the different lithofacies. The observed lithofacies are as follows.
This is the most abundant lithology (~32%) and consists of a poorly sorted medium-grained sand to silt-sized wackestone dominated by sponge spicules, skeletal grains, and foraminifers (Figs. F2, F3). The foraminifers are large and tend to be thin walled and globose in shape. Radiolarians and tunicates are present but rare. This lithology appears to be highly bioturbated throughout with areas that are very muddy, giving a mottled appearance. Quartz and plagioclase grains are rare, and the grain size is fine to very fine silt.
This is a well-sorted packstone representing ~30% of the samples studied (Fig. F2). The most abundant grains are foraminifers (diverse fauna) followed by skeletal grains, many brown in color (Fig. F3). Sponge spicules are common, and tunicates, radiolarians, echinoderms, and bryozoans are sparse. The samples appear to be bioturbated with micritic burrows as well as burrows filled with abundant quartz grains.
This lithofacies represents ~17% of the section and corresponds to a fine-grained, moderately sorted wackestone composed of spicules and foraminifers (Figs. F2, F3). Globular planktonic foraminifers are upper fine, whereas benthic foraminifers are very fine grained, sand sized, and less common than the planktonic foraminifers. Foraminifer diversity is low overall. Tunicates are common, and brown skeletal grains and echinoderm fragments are rare and silt sized.
This lithology is characterized by the abundance of skeletal grains and abundant planktonic and benthic foraminifers (Fig. F3). It represents ~10% of the section measured and tends to be moderately sorted (Fig. F2). Bioturbation and patchiness in grain concentration are common. Sponge spicules are abundant, and bryozoans, tunicates, and radiolarians are common to rare. Quartz grains and dolomite rhombs are present floating in the matrix throughout.
This lithology represents a small (~6%) fraction of the stratigraphic section (Fig. F2) but is unique and very distinctive because of its light gray to white color. The contacts are transitional because of bioturbation. The matrix is dominated by calcareous nannofossils and can be described as an ooze. Sponge spicules are abundant, as well as planktonic foraminifers (Fig. F3). Bioclasts and benthic foraminifers are common. Tunicate and echinoid spines are present. In core, it is highly bioturbated with large burrows infilled with darker sediment. Sorting is poor.
This is the most unique lithology, representing ~5% of the section (Fig. F2). It is unique in the size of the grains (medium to coarse grained) and good sorting. The most abundant grains are foraminifers and skeletal grains (many stained brown). Sponge spicules and bryozoan fragments are common, and tunicates and radiolarians are rare (Fig. F3). The diversity of foraminifers is very good, with globigerinids and miliolids being the most common. Some foraminifers have pyrite enclosed in their chambers. Some parts are mottled and muddy. Resedimented black mud intraclasts are present. Quartz grains are occasionally present in the matrix.
Grain-size analysis showed that the clay fraction dominates (44%; range = 70%-15%) in the studied interval (Fig. F4). Fine and medium silt sizes are the second most important group. As expected, the percentage concentration between medium-fine silt and clay shows an inverse relation. The coarser sediments dominate during some intervals (130.2, 133, 137, 143, and 145 mbsf), whereas the fine-sized sediments dominate during other intervals (127, 131.5, 136, 142, 144, and 146.5-150.5 mbsf). Most of the wackestone has between 30% and 70% clay, and the packstone has between 20% and 50% clay. Coarsening trends can be seen from 151 to 143, 137 to 134, and 132 to 127 mbsf.
The percentage of grains coarser than 68 µm from sieving (see far right column in Fig. F4) fills in the areas with poor Sedigraph grain-size data. An additional coarsening-upward trend is identified in the interval between 143 and 138 mbsf. The coarsest interval in the section corresponds to the lithofacies P3.
Sieved and thin section samples were analyzed to count percent component variations. The components that best characterize the different lithologies are the skeletal and foraminifer grains (Fig. F5). Overall planktonic foraminifer grains are more abundant than benthic foraminifers. Based on previous work (James et al., 2001; Holbourn et al., in press), grains derived from the shelf are abraded and generally are mollusks (some brown) and benthic foraminifers. Grains derived from the shelf margin and slope are better preserved and consist of bryozoans, sponges, tunicates, and foraminifers.
Grain counts indicate that skeletal fragments and planktonic foraminifers are the dominant form, with 42% and 17%, respectively. Sponge spicules, bryozoan fragments, tunicates, echinoids, radiolarians, ostracodes, and some unknown carbonate aggregates are secondary. Skeletal and planktonic foraminifer percentages show a reciprocal relationship (Fig. F5). In general, skeletals dominate in packstone lithologies, whereas planktonic foraminifers are more abundant in wackestones. An anomaly is present in P3, where well-sorted planktonic foraminifers are dominant over skeletals. An additional anomaly is W1 between 143 and 139.5 mbsf, which contains abundant skeletal grains.
Figure F6 shows the stratigraphic section and photographs of the >68-µm fraction. All the photographs have the same magnification, thus highlighting the grain size, sorting, and main components.
In thin sections, point counting of percentage of planktonic foraminifers shows a similar trend to the grain counts (see far right column in Fig. F5). Although the resolution is lower, the intervals with increased planktonic foraminifers in thin section reflect the trends recognized in the grain counts (wackestone vs. packstone), including the uniqueness of P3.
The mineralogical composition determined by XRD shows that LMC and HMC are the dominant phases followed by aragonite as the least abundant of the three types (Fig. F7). Minimal to zero amounts of dolomite and quartz were observed at certain intervals. HMC and LMC show a reciprocal relationship. HMC corresponds to intervals dominated by skeletal grains, whereas LMC increases when planktonic foraminifer abundance increases. One exception is the interval between 148 and 150 mbsf (Figs. F5, F7).
Dolomite rhombs are more frequent around the turnaround between wackestone and packstone lithologies. An exception is present at ~141 mbsf, where a dolomite peak is present in the middle of W1.