Opal percentages in superficial dry bulk sediments and gravity core samples range between 0.2–5.1 wt.%and 1.1–2.0 wt.%, respectively. Maximum opal percentages are found in the inner part of the ria around San Simón Inlet. Values of 2–3 wt.%typify the inner-central part of the ria. Throughout the ria longitudinal axis opal content is about 2 wt.%. Smaller values are found in the margins at the mouth of the ria. Opal distribution throughout the core is irregular, but there is a general tendency for higher values in upper muddy level and lower values in sandy sequence.
Opal analyses were performed for the total and <63 μm fractions of both ria and core sediment samples. For the core samples, there is no correlation between opal content in the fine and bulk fractions, but opal percentage in the muddy fraction is an useful parameter to standardize results and to apply as a paleoproductivity proxy. For the ria surface sediments there is a good correlation between BSi content in both fractions (R2