Preservation of laminations in hemipelagic and pelagic sediments is routinely attributed to reduced oxygen conditions that inhibited benthic activity. As such, the degree of preservation of laminae is used commonly as a paleo-oxygenation indicator. Recent evidence from deep-sea pelagic sediments in the eastern equatorial Pacific and North Atlantic, however, demonstrates that preservation of laminae may occur by rapid deposition of ungrazed Thalassiothrix longissima diatom mats that overwhelm the benthos in otherwise oxic bottom waters. Here, we report that similar diatom mat laminae are common in hemipelagic sediments of the Guaymas Basin in the Gulf of California, have aided preservation of the laminated sediment sequence and have also enhanced opal burial. Examples of diatom mats influencing sediment preservation are also described from the northern California Margin and from the Miocene Monterey Formation, southern California. Thus, even in classical settings of laminated sediment occurrence, caution needs to be taken in correlating degree of lamina preservation with paleo-oxygen levels. Such studies must be underpinned by detailed microfabric and micropaleontological analyses.