Analysis for estimated percent sand in cores taken from San Francisco Bay, California, 1990-2016

Cores taken in San Francisco Bay were divided into the following bays: South Bay, Central Bay, San Pablo Bay and Suisun Bay. Each core was logged in one or several of the following ways prior to this effort: paper logs based on observations of the working/archived core, graphic illustrated logs included in various reports, and grain size tables included in reports and open file reports. The intervals chosen and qualitative findings of sand reported are drawn directly from the paper logs, as they have the most detail in describing units and illustrating sand presence. The paper logs were obtained electronically (some were included in open file reports) or physically in labeled binders at USGS Menlo Park. The other logging methods were used as secondary confirmations of the paper log content. For just a few cores, the secondary logs did not line up with the paper log. These cores were visited in person to confirm the mismatch. Where sand was described in the written portion of the paper logs and/or drawn in the visual depiction of the core, percent sand was estimated. The illustration of the core helped contextualize the distribution and thickness of sand in the units. Several paper logs included a written percent sand estimate, which was then reported in the spreadsheet for this effort. Where there was not a percent sand estimate, various methods were used to estimate sand presence in the units. If a unit contained sand laminae, then they were counted and estimated for thickness from the visual depiction of the core and associated descriptions to determine the amount of sand in the unit. If a unit contained sand interbeds or lenticular bedding, a similar process was used depending on the thickness of each bed. Where there were sand-filled burrows, an estimation was made based on the frequency and distribution of the burrows and their apparent thickness drawn on the paper log. If a unit was described at “Muddy Sand”, “Sandy Mud”, or any other description in which sand was either a matrix or a secondary grain size in any unit and not clearly distinguished visually on the paper log (i.e., sand was noted in the description but not visually marked in any identifiable way on the drawn core), a best estimation was made based on the description. Percent sand for intervals was calculated by dividing the total amount of sand in centimeters from the length of the unit in centimeters and multiplying by 100. Percent sand was estimated for the entire core when at least one unit contained sand. This was calculated by adding up the amount of sand in every unit and then dividing the sum by the length of the core. If a core had any gaps, these were subtracted from the overall length of the core.